Any
IMD from those (and, by abstraction, external units) is probably 50dB
to 60dB down.

And considering that 3rd order transmit IMD from some newer rigs is
coming in as bad as just -20dB, I wouldn't worry about SWR meter IMD!

It seems like the loose coupling of the diodes to the transmission line
would limit the effect to whatever the coupling ratio is as an upper
bound.

If you're applying a signal to the diodes that's already 20-30dB weaker
than the power going to the antenna you're already doing pretty well
compared to, say, an FT-857's transmitter output.

Dan