OK...so way late here, but...
I have used Duplicolor Upholstery and Vinyl coating for a ton of stuff (usually just on vinyl). I wound up giving it a shot on my girth belt and it worked pretty good. Not a lot of colors to choose from, though. To get the dark, blood red color I wanted, I had to saturate the thing (used almost an entire can) in dark red and then mist on charcoal grey. Not exactly what I wanted (and the belt is a bit too small for my gut), so I'll probably replace it and do the stain method. It looks to give a much better coverage and probably has lots of color choices.