Andy,

I don't know those particular mods, but my IC746 was supplied (quite
intentionally) with all band transmit capability.  However, the
transmitter is still inhibited when trying to operate below 1.5MHz as
the PA cannot cope below this frequency.   Presumably the MARS mods you
mention only require transmission at a few other HF freqs so the 100kHz
lower limit could be a misunderstanding / typo on the authors part.

About 10 years ago I started using an Icom IC-735 as an LF exciter.
"Opening up" for all band transmit involves clipping one or two diode leads
if I recall correctly, but different rigs have different inhibit schemes.
The RF PA has basically nil response at LF, so there is no harm done by
"transmitting on LF".  Some rigs allow turning off the PA bias when used for
transverter applications, which is a cleaner way of doing it (no quiescent
dissipation going on).

One way to extract LF drive is to make a "coupler" at the transmit mixer
(which is genuinely broadband).  I used an inductor input low pass filter
for LF (which theoretically goes high Z input impedance above cutoff), and
confirm that the microchoke input inductor had no problem with suckout at an
amateur HF band.  I fed the LF transmit signal out the transverter
connector.

I do not know the IC-746 details, but there may even be opportunity to
install a changeover relay somewhere following the transmit mixer, if there
is a coax lead going to a separate PA module.

73, Bob