Okay, sports fans, mission accomplished.
I placed the girth in a glass bowl, poured a can of acetone over it and let it soak for 5 hours. After this time, the acetone completely soaked into the girth and dried.
Then I hand-washed the belt with laundry detergent. NOTE: Use rubber gloves for all these steps!
I heated a large pot of water to a simmer, poured in cup of salt, half cup of laundry detergent (helps to soften the water), and my Rit dye. For the ROTJ belt, I used two packages of Rit Scarlet (couldn't find Cardinal) and a third of a package of Cocoa Brown. I stirred the mixture, and then added the wet girth.
Keeping the heat just below a boil, I stirred the water for about 45 minutes (didn't have a wooden spoon, so I used a spare DP range finder stalk; it came an interesting color).
Then I hung the belt on the dry rack (board with 4 screws/nails). It appears to have only shrunk by about 2 inches, but I was able to stretch it onto the board easily. The color is now (after drying) a reddish-maroon, exactly what I was after, even if it is not 'dead-on' accurate.
So now...is there anything I need to do to keep the belt from rubbing dye all over my jempsuit/codpiece front? If I flat-out wash it, will it bleed the dye? Is there a sealer for this?