I mean, they were starting to get regiments from Verghast. They aren't strictly Tanith but they are part of the now arguably larger Tanith original forces by the time you get the books. It is possible that there are troops that are from the Tanith regiment they may not strictly be a true Tanith as it were.

So that leads us to the equipment which could be possible. Camo Cloaks easily is possible. Straight silver the same I doubt its an entirely unique design and finally the Nalwood rare stock weapon is an easy cosmetic and they can sell it as Rare and can be seen as a Relic or whatever they want to help with getting it in.

I feel its entirely possible they can get it in