Yeah, I've done them that way - You kind of wedge the screwdriver blade in, and hold it with your third hand, and then usually have to do it a couple times to get it right - Even with a good washer, it takes a couple tries.
These pedals had a pretty deep recess, and it would have been awkward - I just wish that all pedals had the same nut sizes, so you could just buy a tool like this on Amazon for $6, to fit them all - I would have bought one long ago