Despite ABC Playback being touted as Windows/Mac support only, it should be pretty easy to hack up. Looking at it on Windows this morning with the trusty Live HTTP Headers Firefox extension switched on, I've grabbed what seem to be the important URLs, which are XML files.
The actual interface is embarrassingly tedious. They've tried for the latest and greatest in Flash crappiness, which probably means it'll be slow as a dog on a low-end computer. Fortunately, the data is passed through XML, so it'll only be a short time before someone writes a MythTV plugin to grab, parse and play.
There's a channel list, which corresponds to the three channels: ABC CatchUp, ABC Real and ABC Shop. Loading up one of the URLs given for a channel, you get a listing of the content on that channel, including where to get the file. Seems to be you need "rtmp://cp44823.edgefcs.net/ondemand" and then the bit after.
So, despite being annoyed they're blowing our money not supporting all operating systems and instead building crappy Flash stuff, we should be able to bend it to our will.