Well, anyway, installing ThirteenAG's widescreen patch and then reinstalling SilentPatch on top of that, I do have unstretched 16:10 (at least judging by the circular map on the HUD).
It still runs a bit like shit for a game with these graphics though, even on my shit-ass computer. I can see big stutters when running around (even though the MangoHud frame rate display mostly just shows 30 like it doesn't know what the fuck is happening but maybe it just doesn't update fast enough to catch it). I'm too lazy to put the frame timing graph back in to see how it looks; I really just care about how it feels, which is somewhat shitty.
I did, by the way, ignore all the suggested tweaks on ProtonDB, so maybe they're worth something even though I didn't need any of them just to make the game launch.