I would think these to be philatelic, considering almost all philatelic covers are also historical in nature, i.e. First Day Covers, First Flight and expedition types. I think that a certain amount of historical nature can be had out of almost any cover. Although most collectors disdain covers that have out of use stamps, stamp sets, gold plated stamps on cover and Maxi card, others still collect such thing and love them (until they get ready to be sold); nevertheless, they do fall into the collectible, if that is your cup of tea.
I find your covers interesting for probably the same reason you do, the fact that someone in 1941 could make such a trip. Interestingly, I read recently about a couple that were doing somewhat the same thing and even 70 years later, this type adventure is still fraught with difficulties. I just wonder if your two guys actually made the full trip they hope to make. Obviously, they were likely sending the covers to their patrons that were helping finance the journey. I say go for it if you can get enough material together.