Anyone know of/like using a ring modulator - plugin or hardware - that has the functionality of
keeping the modulations within the frequency sets that make up particular keys/scales, as opposed
to the more usual standard freeform wildness?
That's great too, but to have the option for either scenario is really ideal, for me at any rate.

 

Find a ring modulator that has a side chain input. Play a synth with sine waves tuned to your liking as a modulator into the ringmod.

Remember that ring modulator doesn't produce harmonic sidebands, instead they are simple linear additions (or ridiculously complex additions if you use something other than sine waves). Say you have a 1000Hz signal as the carrier and feed it a 100Hz modulator, you end up with 1100Hz and 900Hz, two things that are not harmonically related to each other or the original carrier signal.