I did it! seems like I didn't wreck anything either!

Ended up using silver to make things easier for myself. Mitered the posts using a half-round file to get them close, then sandpaper wrapped around the tube for a precise fit.

I put a blob of silver on the end of each post, then used pliers to place them on while heating (mostly the post) with a torch. The first time around, I let them freeze while the pliers were still on, but in both cases I failed to get the post perfectly flush up against the tube (it's hard to feel and you can't really eyeball it) They were both attached, but not sitting flat on the tube. I re-fluxed, clamped the bike in the stand so that the tube was level, and then re-heated the posts just until the filler melted again (without holding the post at all). The posts just kinda plopped into place (due to the good miter job) and they have a nice silver fillet all around.

Next step is painting!

Thanks MUCH for all the advice, much less scary once I was actually doing it.