if you need to wait 7 days, keep it in the fridge.
what wine yeast do you have on hand? problem with wine yeast is that is can't ferment maltose so the resulting brew will be quite sweet. i'd hold out for beer yeast. this is precisely the reason why i try to have a pack or two of dry yeast in the fridge... you never know.
idea: when your 5 gallon batch is actively fermenting, pour the 1.5 gallons into your main fermenter, and then use a auto-siphon to pull back out 1.5. this mixing will allow you to pick up active yeast.
another option is to put the 1.5 gallons into plastic containers (tupperware, yogurt, etc) and freeze it. later, thaw out and use as starter wort (boil it first). if the OG is high, dilute down to 1.035-1.040, either before freezing or after.