Hi again, and thanks for the reply.
The Logs folder remains empty no matter what I try - I changed the debug settings on and off, but nothing is ever logged.
As a further test, I downloaded the most recent version and tested it with JPCSP (set to PSP 2000) - still only getting the internal bios, and a black screen after that. I tried discarding my settings file and starting over, making a single change to the default (pure CGA instead of VGA), but the result was no different. The same configuration works fine on the Windows version. A test with the ET4000 rom yields the same results.
Since JPCSP works for you, and the settings are (nearly) the default ones, the only option that I can think of is that the CGA/ET4000 bios files that I found are incompatible with the PSP for some reason. Have you tried your configuration on PPSSPP?
As an aside, I managed to recover the source code for Dosbox PSP and uploaded it here - it compiles and works, but the keyboard is pretty broken. If you felt like adopting the project it would be the greatest thing.