I think you might be able to do this with an IR Translate in Hijack.

With Hijack installed, pressing and holding the front panel knob-button will normally toggle between sources. I seem to recall that by default, it only toggles between tuner and MP3, but there is a configuration tweak that lets it toggle between all three sources. I don't remember if that configuration tweak needs to be in config.ini or if it's in the Hijack main menu, but a bit of research should reveal that.

Then all you need to do is research how to do IR Translate commands, and have the IR for the source button execute the internal command for the hijack front-panel-knob-button-source-switch, whatever that command happens to be.

All of the above will likely be findable in the Hijack section of the FAQ, it may take a little digging to find all the relevant bits, post back here if you get stuck.

Let us know how it goes.