kvest,
The ladies are doing a good job of talking through dictation with you. One thing you will discover as you journey further and further down the homeschool path is that when using an HOD guide it is really important (before changing an assignment within the HOD Guide) to understand the purpose behind the original assignment. With that in mind, if you take a look at the dictation assignment, you will discover that its purpose is for kiddos to learn to visually capture a passage in their mind, repeat it back to you from a single dictation (while holding the original passage image within their mind), and then perfectly reproduce that passage as they visualized it. They then check that passage against the original, teaching them proofreading skills (not for the purpose of finding an error within the original passage but instead for the purpose of finding and correcting an error in reproduction within their own passage). This ability to visualize, reproduce, and proofread has a powerful impact on a child's spelling and writing capabilities. It later allows them to capture and remember entire passages, to easily absorb and reproduce a writer's style, to begin proofreading their own work, to be able to think while writing, and to write purposefully.
So, in thinking over the purposes for studied dictation that I've just shared, which are mainly focused on spelling and writing ability, if you look at the turn you are taking with the assignment you will notice that the assignment has now become a grammar teaching moment. This means that the main focus has turned from a child-focused visualizing process to a teacher-focused grammar lesson (also focused upon finding and correct errors). In taking a turn like this, you are taking away the intended focus of learning spelling and writing skills through visualization, and instead are functioning in the realm of what we expect Rod and Staff English to do (in emphasizing grammar, punctuation rules, capitalization rules, etc.).
One thing to bear in mind is that each segment of the HOD guide has its purpose, and when you change an assignment's purpose you will often find yourself overlapping into areas that we already have covered in other ways. In my opinion, there is no better grammar and writing program than Rod and Staff English. Through using Rod and Staff, you will find the grammar, punctuation, capitalization, and usage lessons that you are desiring within that program as scheduled within the HOD Guides.

My own oldest 3 boys have all done the studied dictation within our guides alongside Rod and Staff English. My boys write very well and are not in doubt at all about the rules of grammar, punctuation, capitalization, and usage. They did sometime comment about a missing comma or capital letter in dictation, as per Rod and Staff rules, however they knew the focus of dictation and knew it wasn't meant to be a grammar lesson. Since the dictation passages come from old dictation texts used in public schools in America, you can imagine that over time some of these rules used within the structure of the passage have changed. You'll also notice that some rules can be in flux at times in history, not actually being clear as to when to do what with a sentence.
In closing, I would urge you to use Rod and Staff as your grammar teaching moment and leave the dictation passages to do their intended job as written. In sharing my reasons for suggesting this, I will share that during my 11 years in the public school (prior to these last 13 years I've been homeschooling), I did exactly as you are suggesting, by writing a passage on the board daily for the kiddos in my class to use in finding grammar, punctuation, capitalization, and usage errors. We then corrected it as a class together, giving a reason for each correction. On Friday, my kiddos had a quiz over those same passages and were expected to find those same errors and fix them. Through the years, it always surprised me that there wasn't better retention on those Friday quizzes. It seemed that those who were naturals at grammar or very rule oriented did well, and those who weren't did very poorly. This exercise also did not seem to do anything to raise the kiddos test scores in this area at year end on standardized testing, even though we diligently spent time daily in this grammar exercise.
When I read Charlotte Mason's philosophies and ideas as a home educator, I finally realized why this was so. By doing this type of grammar lesson focused on "fixing mistakes," I was actually allowing the children to visually capture the mistakes. They, in essence, were so focused on the mistakes that the mind captured those as being important, rather than visually capturing the correction. It would have been better not to highlight the mistakes.
I know this type of thinking can take some time to internalize, but weighing the intended purpose for each assignment can make all the difference in how you teach, guide, or grade that assignment. While we could have written our own perfect grammar-rule oriented passages for use in studied dictation, in doing so we would have lost the wonderful poetry and passages from literature that do not follow these perfect rules. In truth, many of the great writers choose not to follow all of the known conventions of writing in their works. Can you imagine editors going back and applying all sorts of grammar and writing conventions to famous works? In this same way, we have let the dictation passages stand as they have stood for over 90 years. While it's true not all of the passages contain great writing by any means, the question arises if you begin editing the passages where do you stop? When the passages do reflect poetry or great writing, do you fix that as well, even if the author didn't write it that way? I"ll leave you to ponder, as I know I have too!
Blessings,
Carrie