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