There are no known "hacks" for the R15, I would suggest getting a Tivo-based model (can be had for around $50 online) Then there are a ton of solutions for Mac OS X to get shows off, stream shows from your computer, add tivoweb and a host of other things. if you have a PC there are even more options.
The R15 is DirecTvs try at a DVR, I had one in the spare room for 4 months before I gave it away, to me the TiVo based DVRs are sooo much better overall.
They did make the R10 which was TiVo based but I wouldn't get that as you need to solder the chip to hack it, a Hughes HDVR2 or similar is probably the easiest to setup.