Picked up an older cabinet with metal coin mechs. I wanted the kiddies to "feed the beast" rather than be on free-play ... it's part of the experience, after all.
I removed the lower cover, then the thin spacer (on a second mech, this spacer is more substantial, but the method still works.) From a scrap of 1/8" aluminum, I made a ramp that redirects tokens to the "acceptable" outfeed slot. Dimensions aren't critical - just clear the little floppy tab between the two outfeeds. Put everything back together, using a 6-32x1/2 screw to secure the ramp in place. Works like a charm, and is completely reversible should you decide to revert to normal operation. On one mech, I did have to reduce the thickness of the ramp with a file, but that was really minor. Slightly thinner aluminum (2.5mm or 0.093") would probably work, too.
Note - this only works with quarter-sized tokens. Pennies, nickels and dimes follow a different reject path, and will continue to be rejected. Also, make sure your tokens will fit into the infeed slot - some of my mechs trap the tokens right at the ingress point, and they'll be rejected when you hit the reject button. However, if the tokens drop all the way through, this is a mostly-painless method of getting them to work.