Continuous shots at one setting w/shutter held, one different shot on release?

[telekinetic]

Brand new ML user, just installed on my 7D.

I am a sports shooter, and am hoping to be able to create a rear-curtain-flash-like effect without flash, since flash can't be used at events.

I've looked at bracketing scripts, and I think they might be a good start, but I haven't found anything exactly like what I'm looking for.

Is it possible to set up one of my custom modes (C1/C2/C3) such that while the shutter is held, it takes a series of shots at, say, 1/8 100ISO, then when you release the shutter it takes one more at 1/320 3200ISO?  I plan to blend these in post to create a motion trail ending in a sharp image.

Thanks, and I apologize if this is extra-basic stuff or the wrong forum, I've been lurking and reading for years but finally got my hands on a 7D to play with.