I think that could work. I sometimes keep envelopes for exposed and unexposed paper in my changing bag and carry it.

I thought of two more things:

1) If you are shooting paper, you can aim to make your pinhole slightly smaller than "optimal" because most of the pinhole calculators use "daylight" and the paper will be more blue sensitive.

2) If you use a little water when you are sanding you can avoid breathing the aluminum dust.

And most important of all: have fun!