I think you could get it to work, but it's going to take a little bit of tediousness. You'd essentially have to set up patches as dual tones where one input is muted depending on what channel you want active at the time. So, for the patches that you want the AC30's channel 1, you'd use Input A (set to "guitar") and mute Input B. For patches where you want channel 2, you'd do the inverse - set Input B to "guitar" and mute Input A. As long as you have the effect loop block as the last thing before the split in the tone, you'll maintain separation going through the effects loop. The other thing you'd have to do, though, is split the signal coming from the AC30's effects send so you can have something going into the POD HD500's left and right effects return (otherwise, you'd use one of the channels). Then you should be able to just take one of the 1/4" outs from the HD500 to the AC30's effects return. I think this should work.
EDIT: Actually, now that I think about it more, I think you could be OK without using the splitter in the amp's effects send. It would be OK as long as you don't have any true stereo effects blocks in the post position, but given that this isn't a stereo setup, I don't see why would. Even if you did, they would still work, I guess. You'd just lose one side of the effect.
With a Y splitter cable for the FX Loop, other options are possible.
- put a pan control in front of the FX loop and assign the Pan to the volume pedal .... that way you effectively mix from one channel to the other.
- put two pan controls before the FX Loop, one panned left and one right. Then use a switch to toggle between the two.