These external boxes/gadgets don't use IR. Have a look at the tech and prog sections and there should be info on building and programming control devices. From the information on the paged that you listed this project should be cake for some of the folk out there.
A decoder chip to sense the resistance on the wire, a few plugs and a small bit of programming. I'm no elc. eng. person but this seems to be quite easy for those who know.

Murray 06000047
I don't think, therefore I am not.