I don't see any scenario where that would work reliably. You'd have to scrape up some of the goop, weigh it to try to be precise - except the weight would be different after absorbing liquid (how much, who knows). Buy a few more oz of hops and save yourself some grief, dude.
EDIT - Not to mention that a lot of the varieties that get dry hopped have great flavor and aroma but are lower in AA or are just less than stellar as a bittering hop.