I kinda thought so; either that or a big-ass reflector. I used to have a big, heavy projection screen with the silver glass-bead surface and a weighted base that I used for location fill. I picked it up at a flea market for $5 in 1984 or so. I miss it sometimes, but I don't miss carrying it!
I still have some umbrellas and lightstands (and strobes, but they don't help on location
), and I recently got the brackets that hold a shoe-mount flash with umbrella. Do you use manual flash or P-TTL for fill like that? I mean, I've got a Metz AF 48-1 and it works 'off-camera wireless' with the K-5, but I've not played with it in fill-flash conditions.