got it. here are my additions to xmamerc

ror
fullscreen 1
heightscale 2
widthscale 2
autodouble 1
sdlmapkey 0x0073,0x000f

the last one maps unix keyboard key_s to sdlk_tab. I had previously mapped the start button on the bluetooth gamepad to s.

so if you wanted you could map a hardware key to tab

fab!