I've seen almost no legitimate signed T boxing cards.
The most likely prewar boxing cards to find signed (besides self-issued promos and souvenir restaurant postcards) seem to be Exhibit cards. I've seen several Dempsey, quite a few Schmeling (modern signed since he lived to 100), several Sharkey (career-contemporary and much later), a few Tunney, and a smattering of non-title and lower weight classes.
Signed prewar strip and insert cards are rare. I have two and each appears to be signed decades after issue
I just don't think these guys got hit up for autographs like baseball players, unless they were in the hospitality business.