The multipart purchase that you describe sounds like a good idea. The only situation in which I can see it not working is if there isn't enough room between the clew of the sail and the end of the boom for two blocks and two sets of shackles, and of course the line.
I have a larger boat (with a mainsail of 136 sq ft) that I used the purchase that you described until I got a sail that is longer on the foot. There wasn't enough room for both blocks so I attached a block with two beckets to the eye in the end of the boom and ran a line from the other becket forward, through the clew grommet and back through the block, then forward to a clam cleat on the boom.
I found a line tensioner in a junk box in a chandlery that I hooked up to pull the line tighter through the cleat--the tensioner has enough mechanical advantage so that I can tighten the outhaul under load.
Instead of a line tensioner it would be possible to rig a multi-part purchase along the boom itself, which would pull on the outhaul line after it had gone through a block at the end of the boom.
Regards
Tom Grimes
P-14 #363 Far Horizon
Muncie, Indiana