leather double welt?

[Darren Henry]

It's a small stationary upright sander.----It did work quite well on the scrap I tried, but  I will have to be very careful not to twist the band of leather

Just re-read your post and caught the fine print. My bad. If your sander has an adjustable "table" to run the project over like a wood thickness planer, you're good. Pull with one hand and feed with the other like using a splitter. Otherwise make your support board as long as a comfortable stoke and hold top and bottom as you draw it past the belt. Swipe--reposition---swipe---repeat---