Hello Ben!
I have nothing to add to what the others contributors mentioned. It's really a pity that those particular letters were lost; another insider on the Portuguese army would be great. About your quest on personal memories by Portuguese military men, it's a dead end. In the sets of memoires written by a few Portuguese officers, those that latter on the XIX century had relevant political/military roles in Portugal, the Peninsular War his only a footnote, they were all young officers. After the end of the Peninsular war, most of the officers in the Army got involved in the political conflicts which led to a major civil war that divided the country and other minor internal conflicts, expeditions to South America etc. We can imagine writing his impressions on the Peninsular war was not a priority. We have some references to sets of correspondence buried in family archives and at least a journal, only very partially published in the work of Chaby, that's all.
Hope it help.
Moisés