Hi bitbytebit,
I've been doing all kind of tests editing mame.ini, all opengl options I've imagined, filters, screen, turning off vsync, but the issue is still there. As you said, these eselect commands are failing for the readonly file system. I also have a problem with this version not being able to get in ldex when I try to activate eth0, it just gets stuck in a black screen after the config terminal exits and I have to turn off the machine. That also happens if I try to mount a drive. I believe this stuff with the drives, not the network, has been there from previous versions, and may be the reason that I haven't been able to mount a drive yet, but at the beginning I was confused with the other issues, if only I had more experience with this system I could provide you with better feedback. So I keep stuck with this, it could be something silly with my machine, though I never had a problem before, however these days hopefully I'm going to test this on the AVGA cab. As I can't get the network working nor plug a usb stick I can't send you the logs, I've copied by hand some of the output of switchres invoking mame.
Here it's reporting an audio error (btw I haven't been able to have audio working with any live-cd I have tested, not only arcade gentoo). This shouldn't be the cause, but just in case:
Audio: Start initialization
Audio: Driver is
Audio: Initialization failed. SDL error: No available audio device
output: unable to open output notifier file /tmp/sdl_out
[...]
OpenGL: VBO supported
OpenGL: PBO supported
OpenGL: FBO supported
OpenGL: using vid filter: 0
GL texture: copy 1, shader 0, dynamic 1, 256x240 256x240 [PALETTE16, Equal: 0, l: 0, Palette, 1,
scale 1x1, border 0, pitch 320,256/8192], colors: 2048, bytes/pix 4
... that's the end of the output, right after there it dies.