Bernard Cornwell, in reviewing Rifles said something similar to you - that he wanted more on the development of the Baker weapon itself. I see that it is an absolutely fair criticism. Of course, this is my judgement in hindsight.
I'm sorry to disappoint, but I won't be writing that follow up you crave. I would say two things about this. Firstly, my approach is to try and give a subject a really thorough going over and then move on. I have not, for example, tried to write about Napoleonic codes again following my previous book. Secondly, when I research something I can only do it by retaining a very tight focus. I am struggling to complete my research to a tight deadline while bringing up kids, holding down a job etc. So that means I could only offer limited additional information on the subjects you refer to. Always, in the research, there are difficult questions of prioritisation. Should I, for example, have made greater efforts to track down the descendants of Capt Jonathan Leach, so as to have had access to all of his unpublished MS journal ? I had 75% of it, from Verner's notes, but a key bit was missing. I made some inquiries trying to find the family (who still had it in 1910 when they allowed Verner to borrow it) but gave up.
If there were to be spin-offs from Rifles, I would have two in mind. One would be a printing of Leach's real journal - nicely annotated to highlight where he self censored in his published works ! The other, which I've considered, is the 1st/95th nominal roll for 25th May 1809 with explanations of what happened to ever soldier on it. I would envisage the last not as a book, but perhaps as a web project.