Since we really are looking at exploiting Onslaught's ridiculous weakness, are there enough members of this "JLA" (Black Adam? Really?) to absorb all of Onslaught? There were a lot more Avengers and FF members than that, and Pym must have counted for quite a few on his own, given his size/mass.
They were a mostly low powered bunch by contrast though, some of them basically straight up joe humans at that. I get the sense that fewer, but vastly greater in power overall should soak the dude up Bounty style just fine.