So, just as a followup on this: after some trial-and-error with STI to get the scaling figured out, I finally managed to get it so the number of tiles onscreen would be locked and... it looks like crap. The art assets simply don't scale up well, even at even increments. If I had gone for a more pixelated look I imagine it'd be fine, but the sketchy style ends up turning into a blurry mess when you go above native resolution.
So, it sucks, but I'm making the call to let the camera show more than intended if you have a higher-res monitor. Definitely something for me to keep in mind next game jam though!