For a time I used a studio rack compressor in the effects loop, to compress all the signal all the time. This was with a Markbass head, and I had to set the effects loop to serial, otherwise half the signal will bypass the loop.
Other than that, I can't see the point. Most effects produce instrument-level signal rather than line-level signal, and are designed to go through the pre-amp gain stage. Some work in a loop, some don't or fail to do what you expect.