AFAIK, hops won't contribute significant bitterness or desierable flavor if just boiled in plain water. The sugars of the malt extract or AG runoff (what's the word?) help to ... isomerize? (or similar) In fact, it might add some undesireable grassy flavors.
So, it's a waste of time, unless you plan on actually boiling your batch up in two 1/2 batches, which I'd think is a waste of time. B/c you'd want to boil your second part for a full 60 min, instead of whatever you're doing for your top-off H2O.