Hello, and welcome to the forum!
There was a post in the Ideas Exchange regarding RMVB support, you can view it here:
Unfortunately, its status is currently " Unplanned". The reason is as follows:
Unfortunately with this specific hardware platform, RMVB codec cannot be supported. However, WD does offer the WDTV Mini Media Player which supports RMVB decode.
Perhaps you could stream the RMVB files to the Live over the network from a PC / media server like TVersity? You'd need to enable transcoding (normally you'd disable transcoding as the Live handles most other codecs fine) but I think it would work (in theory!).
If you don't have your PC running all day anyway, it's not an ideal solution.
Can you link to a sample RMVB file that is representative of the type you'll be wanting to play? If I get time, I'll test out my idea and let you know if it works.