Another thought I had...Ranger carries a product called Cut 'n Dry which comes in a sheet of
either felt or foam - I have the felt and love it! Basically, you cut it to whatever size you want, ink it, and you have your own custom stamp pad. It's the exact same stuff as in a regular stamp pad, but the advantage here would be you could just cut a "spot" size square and use your white Craft refill as a test, and not have to ink up an entire pad (like the one you'd buy at Michael's), and if it doesn't work, you wouldn't be using up as much refill. And if it does work, you've got the rest of the sheet to make othe pads from. Anyhow...just another thought.
Here's a link to the page that explains it:
http://www.rangerink.com/products_drypads.html
Good luck, and let us know (if you do try it) whether or not it works -- I'll be next in line if it does.