OK, I think I've solved my problem. Not entirely happy with the outcome, but I think it'll work:
Those of you who followed a question I posted on Natter and Bitches about what might make a 19th century woman logically conclude she was infertile may remember I moved Lucy from 1810 to 1809 to make the 1811-12 setting of Anna be two years after Anna's marriage to Lucy's cavalry officer cousin Sebastian. At the start of Lucy, Sebastian is away from his regiment recuperating a leg wound that's left him temporarily unable to sit a horse. When I moved the setting to 1809, I thought it would work perfectly to have him wounded on the retreat to Corunna, since all the troops involved were evacuated back to England anyway, returning (so I thought) a few months later. All I'd have to do is make him not well enough to rejoin his regiment right away.
Except I remembered after already starting my rewrite that I really needed to do due diligence and make sure the regiment I'd picked for him was more or less where I needed it throughout both books and the two years in between. That's when I discovered that, best as I could piece together from brief online regimental histories and lists of battle honours, the cavalry regiments that were at Corunna, unlike the infantry, didn't return to the Peninsula until 1812 or 1813. Cue writerly angst. I didn't want to make Sebastian an infantry officer, though I could if I had to. And I really didn't want to invent a fictional regiment, because it just seems kinda sloppy to put Jack in a real one and not do the same for Sebastian.
So. After squinting at the list of battle honours for awhile, I noticed the 16th Light Dragoons were absolutely everywhere I needed them to be BUT Corunna. Poking around Google eventually revealed they arrived on the Peninsula in April 1809 and stayed for the duration. Problem is, Lucy opens in May 1809. My current idea is that Sebastian sustained some kind of accident (broken leg?) or illness around February March 1809 that left him in no shape to depart with the rest of his regiment. It changes him from "romantic wounded veteran" to "prickly and somewhat embarrassed at being absent from his unit at such a time," but I think my plot will still work. Right?