Your best option is to create the key for each original clip, then export that out as an Intermediate and use those for the actual editing.
I agree with Jim.
However, you can copy a clip that has the key that you want, and then paste the attributes of that clip on to the other clips to save time. If you select all of the clips you want to key at one time, you can apply the attributes all at one time.
That may or may not work out for you as well as just keying them all out one long clip at a time.
Another way to avoid the DI is to put each clip in its own sequence, key it, and then use that sequence instead of the clip when cutting.
As for the clip underneath? No. Sorry. You can do that in After Effects but not Premiere Pro as far as I can tell.
Thanks to Jim and Steve ... very helpful!
I tried the following and it seemed to work well. Key each clip in its own sequence (there are 3), nest each adjacently (make sense?), then multi-cam the nested seq.
The advantage to this, seems to be its simplicity and the fact that backgrounds can be changed easily.
If it works...
As Jim says...
You see, the thing is, one of the best things about Adobe software is that there are often many ways to accomplish something. They encourage your creativity. Sometimes that can be a bit frustrating, but it works out, for most people, eventually.
It should work fine, depending on system specs and source format. I edited multicam sequences of GS ultrakeyed footage before with CS5. Worked great with Canon XF footage.
You might find that matting each source sequence separately, rather than in the MC sequence, looks more natural because that way the backround matte can change perspective with each cut to a different angle in the MC sequence.