They would still have to pay back whatever welfare/schooling/education/etc. they had recieved in the form of fines and could not become citizens until doing so. In a since, they would be on parol. And just like parol, if they skipped town, committed crime, etc. they pay. Instead of jail, they go south. Any stuff they have here is auctioned off by the government or given to charities.
It's the best idea I can come up with.
The "few" to who you're referring aren't actually a small number. What you've basically described are the categories of Illegal Aliens who've been targeted by Section 245(i) of the US Immigration and Naturalization Code. While this has currently lapsed, during several previous incarnations from 1994 through early 2001, about a million Illegals sponsored by family or employers were able to "change status" after paying a $1000 fine (fee).
When Section 245(i) came up for renewal this Spring, the Bush Administration's estimate of the number of the next group of Illegals that would be Amnestied was 200,000. The actual number might be a bit more, though I don't think it would have been "in the millions," as some have suggested.
Of the million Illegals Amnestied earlier under Clinton (this is part of his legacy), those who were sponsored by employers counted against our annual immigration caps. That means that a corresponding number of law-abiding applicants waiting to immigrate to America were not admitted. They were penalized so that Illegals could be rewarded. I don't have that precise number, but I think it's reasonable to assume it's a few hundred thousand of the million Amnestied, probably not a majority.
Those Illegals who were sponsored by family members did not count against our immigration caps, they simply busted them, as is the case with the chain immigration of "family reunification."