Okay, I tried a few things here.
First just updated the midori on my pi from the repo. Unfortunately I don't remember what it was before, but the new version in repo is 4.3. This seemed to "improve" things; GMusic gets a little farther in HTML5 mode but still gets stuck on the loading music step.
Second, I tried installing from Bodhi. I get this error after adding the source above and trying to update:
W: GPG error:
http://packages.bhodhilinux.com debian InRelease: File /var/lib/apt/lists/partial/packages.bhodhilinux.com_bhodhi_dists_debian_InRelease doesn't start with a clearsigned message
Not quite the error I would expect if it's just a key issue, but I could try to apt-key the Bodhi key if I could find it...
Third I built midori 4.7 from source on my Pi and tried that. This gets further still, with the music loading succeeding but nothing happens when I try to play the music. Also it crashes sometimes. And the Grooveshark site above crashes it reliably. I haven't rebuilt with debugging symbols yet but maybe I'll try that. I assume 4.7 just doesn't have enough HTML5 support yet. Or perhaps it's an issue with the WebKit libs that are in the current raspbian versus what's available on Bodhi? I may try building a more bleeding edge midori too.
In any case, the whole thing is still too slow for my taste (running midori inside matchbox so I have minimal window management). I still lean toward some kind of console solution. I'll get to it when I'm done with Pandora/pianod futzing and my LIRC project...
Ideally would using the unofficial GMusic API to add the URLs to my MPD queue. The big downside is that AFAIK there isn't a way to get MPD to read the tags from the stream before it starts playing the stream, nor can you set the tags in MPD from some separate source like a separate query to GMusic. So without patching MPD I don't think I can get the GMusic URLs to show up in the queue with meaningful names?
