Have you heard of EndPad? Basically it allows you to set the end padding for all recordings as far as you want, but it does it on-the-fly, so if the padding would interfere with another scheduled recording, it won't do it.

http://www.tivocommunity.com/tivo-vb/showthread.php?s=&threadid=136658

I've been running it for six months now and it has proved to be reliable and very useful.