Hey Jay,
That's exactly what I did. I put Marx bases on the Deetail figures and painted them in the Marx colors. I also converted a bunch as well. Once upon a time, painting over a two dollar factory painted Britains figure would have given me pause. Now with new issue figures going for a buck and a quarter or more each, it doesn't bother me a bit. And sometimes you can get a great deal on a bunch of Deetail figures on eBay.
I've done the same thing with figures by all sorts of maufacturers, including Airfix. Check out these pages to see better shots of some of the figures.
http://playsetaddict.com/ultimatCuster.htm
http://playsetaddict.com/Ultimateblueandgray.htm