Next time I have my STF connected I will try this, however, it should be possible to deduce what can be done from the research I've done into the GLUE state machine. So, I'll wager a guess that they remove the right and lower borders. Top and left I don't see how that could be done - the mono line start timing is what's used for left border in the other screen modes and Alien documented mono screen start to be the same as 60Hz screen start (i.e, top border position in 50Hz).
If so, it's done by _not_ being in mono at cycle 164 on each line (and letting HBLANK at 184 take care of line end) as well as making sure not to be in mono at the end of (mono) line 400 (should correspond to ~212.5 in 50Hz timing-speak).
The latter position I'll definitely research since it's not documented in the state machine at the moment ("mono screen end") - and the likely accompanying "mono vblank" as well.
/Troed