I know there's other, maybe older and simpler DMM's that have been hacked, and people even used modern MCU to control them. IDK about all the other chip's in there, but something like a 5.5digit Keithley 197, I've seen the full program for, and I think most the parts are swap-able to something obtainable.
I'd say some of the early handheld DMM's would be a fun project too. And for sure, there are open-source DMM projects, even up to a 8.5dight DMM, or was it 7.5, but it was really impressive, and open-source, there's video's of it on youtube.