Wish there was more on the Flanders Campaign; after all, that was the last time that the Austrian Military Air Rifle was deployed in large numbers to the field.
It doesn't surprise me at all that some loyalist French officers made it to the Duke of York's army but I can't recall any mention of this in what I've read.
What I doubt is that any French officers were actually brought into the English army and given an officer's commission; at least I've never seen any indication that this might have happened. Pretty darn sure that such a thing never happened in the English army circa 1801-1814. French officers commission in Wellington's army? Think we would have heard about that by now.
It seems to me that you've changed the subject from French officers being given an officer's commission in the English Navy/Army to did foreign troops serve a long side English. Of course, there were foreign troops such as the King's German Legion that served in Sicily, Spain, Waterloo; but these were separate and distinct from regular English units.
Having foreign officers commissioned in armies was nothing unusual for Russia, Austria, Naples; however, it is, as far as I've ever seen, extremely unusual for the English to do this. As I have already provided the proof of, Nelson clearly stated that he would never allow a single Frenchman, other than as a prisoner, on his ships. Collingwood was just as anti-French as Nelson. So, it seems to me, that there is just no chance of any Frenchman serving in any capacity, let alone as a commissioned officer, in the Royal Navy. There is only one known exception to this: Sir Sidney Smith.