Final Cut Green Screen Question

I am making a video where my friend blows my head off (heh)... there are 2 ways of doing this. One is to take a still picture of the backround and place my video over that one and crop my head out. The other way (and the way I'm unsure of) is to use a greenscreen. I am familiar with how to use green screen, chroma key, and garbage matte but how the **** do i crop out just one portion of the key? Can you put a garbage matte on just 1 portion of the video instead of the entire thing? I'd like the final product to look something like this YouTube - Head explosion effect in final cut better version

any help would be appreciated.

I'd think what you'd do is this:

Film the background with no one there, camera set in position like it will film the actors

Film the actors doing everything up to the head exploding

Put the "green" item over your head and film the rest.

Now in post, put the clips on the sequence so below the section where your head is green, the background only clip is playing
Above the 2 clips, starting from the clip where you head exists to where your head doesn't exist do your "expoding head" so that during the peak of the explosion you transition from clip w/out green to clip w/green so no one sees the vanishing head.

You'll have to work on timing so it doesn't look like 2 clips and that might be a challenge.

At least, that's my first thought how to do it with a green item over your head. There are, I'm sure, better ways tho.

put a green item over my head? like... wrap my head in green tape? lol, is that what you mean?