Yeah, that is doable. You'll just need to get the codes for that buton, eg. what address and command code that buton has. That can be both difficult and easy. Best is if you know the code, if you don't, then you might (hopfully) have an IR-reciever for the computer. Then it's a piece of cake. Coz then you just hook it up to winlirc (or lirc if you use linux) and just get the code through there... Else you can build an ir-reciver (costs like max $5)..
there might be other ways but these are the ways I'm sure of.. and it's really cheap (and easy) to build an IR-Reciever for the serial port.
so... get the codes and then you just change some code (it's quite easy, it's just to change some constants in the actual code) or you ask me if you don't know how to do it and i'll show ya!
so if you have any questions, here is where you find me!
good luck!
/pablot
p.s.
For anyone interested... I still haven't cleaned up the RC-5 code, but it will come soon. If anyone needs it, give me a post and I'll get to it.
d.s
*edit was some typos*