Oh, I see. I just make it easier on myself and not shoot under those conditions.
Yes, if you have to, fill-in flash may be the only way. I recommend getting a Lumiquest pocket bouncer. You attach it to your flash and you can insert a gold reflector, which can warm up the lighting. You use it with your flash pointing straight up, so there is some diffusing. I suppose if you were creative, you could make one yourself, but it's not too pricey. Here is a picture of it:
http://www.bhphotovideo.com/c/product/544387-REG/LumiQuest_LQ_101_Pocket_Bouncer_for.html
I forget if they give you any inserts, but you can always make that yourself with gold card.