It doesn't even have to be trying makeup himself, it can be "I want to learn to put it on other people, will you be my model" or even "I want to know more about it generally."

I do think we're being a little unforgiving of him not knowing exactly how she feels about it (or not mentioning it in the letter, that's not the question he's asking, after all, and it doesn't seem to be a big part of her life, if it weren't for his interest it wouldn't be any more important to the relationship than how she feels about model trains). And if you're worried about accidentally giving insult, there really isn't any obvious casual way to find out what she thinks. But bringing it up in a context other than "why don't you wear it" is the starting point. It could also be something like "I want to buy some makeup for /female relative who wears it/ for a gift, do you think that's a good idea?" - express interest in makeup in a context other than the relative attractive of women who wear it, and see where that goes.