I've done this successfully a few times, and this is what I've noticed:
1. You absolutely MUST inform the juco or he won't take the redshirt without problems. Inform and get him to sign and he will every time just like a freshman would.
2. It seemed to me (no detailed analysis to confirm it) that it didn't take any more or less effort to overcome the inform with the jucos than with the freshmen.
3. This can be a great recruiting move for jucos with a lot of blue ratings and/or high high blues in key areas. Get him to improve and then still get all your seasons out of him.