Luckily there is a bathtub installed on the opposite side of the wall from my darkroom, so I was able to cut a hole in the drywall and tap into the existing piping for the sink connections. Note: do not try this at home. I am not a plumber, nor do I play one on TV. I didn't even stay at a Holiday Inn Express last night.
Here you can see the original plumbing, viewed through the 6x9 inch access hole in the drywall. On the right of the picture is the bathtub and on the left is the exterior wall. The space is very tight and there is no 'slack' in the pipes for inserting fittings.
Here you can see how I tapped into the copper water pipe and the PVC vent stack. Doing this makes the bathtub "wet vented" which may or may not be acceptable according to best plumbing practices, but which seems to work perfectly.
Soldering in this space is very dangerous, especially since there is sawdust everywhere and an ember might fall down the exterior wall. I used floor tiles, wet rags, and aluminum foil as heat shields for the soldering.
Here you can see the mostly-finished soldering job and the cold-water ball valve
The finished result. When I wish to uninstall my darkroom, I can unscrew the drain piping, turn off the valves, and install the access panel door.