I haven't had any real issues getting Myth up. I'm definitely not using it to its full extent right now, but getting it to playback live tv wasn't an issue at all. I just went through the Myth Setup, then launched TV.
How do you get to the Myth setup? When I go to the Myth link under the wizards panel on the left side of the web administration tool, but the panel where options are usually displayed and set is completely blank. Nothing. All the other wizards have something in there, even if it isn't much. But the Myth wizard has nada.
Here's the thing though... since you've set up your Pluto box as Core only, you have no Media Director (MD) functionality there. And since you aren't using Pluto's DHCP server, you can't set up a diskless MD. This means that at this point in time you're probably unable to play back media of any sort, which includes MythTV.
Any idea why it would be that an MD requires the Pluto core to be in charge of DHCP in order to have diskless MDs?