now, I've heard this was perhaps done with some of the older large format cameras way way back, but I was wondering if this could be managed with something like an Ikontar or Bessar camera?
I was thinking that just that smidge larger than contact (say, fit onto a 10x15 postcard) would be nice.
No particular reason you can't do it with any camera if you can arrange a method of getting light through the negative, and the lens can focus closely enough for the print size you'd like to make. Something close to 1:1 might be stretching the focus capabilities of the camera, unless you can extend the lens mounting somehow.
I've seen a few pieces on making enlargers from cameras in some old photo magazines and hobbyist books I've run across.
Graflex made them as well for the 2 1/4 x 3 1/4 cameras. The Graflarger attachments used to show up fairly often on ebay, I suppose they still do, though I've not looked for a while. I have one with a broken lamp, and Aristo told me at the time they still have replacement tubes available for these.
FWIW.