The problem turned out to be I didn't build etherboot 5.2.2 properly, in addition to editing the src/Config file I had to edit arch/i386/Config and switch to the CFLAGS+= relating to linuxbios. In etherboot 5.0.10 the only config file (that I know of) is the src/Config one.
I did try reading the docs, but there was very little mention of linuxbios in the etherboot docs. This whole thing could have been avoided if etherboot had a top level README.Linuxbios or something like that.
The whole point of this exercise was to be able to get etherboot to output to the vga even without a normal pc bios. I tried the CONSOLE_DIRECT_VGA option Eric mentioned, and it works as needed.
There is still an issue with my epia-m vga init code that doesn't set the sync for composite/svideo properly. That means in order to view the output of etherboot you've got to plug in a vga monitor, which we want to avoid. The epia-m vga init code sometimes detects that a monitor is plugged into composite/svideo and automatically switches to the appropriate sync, but this appears somewhat random.
-Dave