The ribbon in a mic may have very low mass, but (at least with the classic ribbon mic design) it's also very loosely sprung so it can move easily. The resonant frequency of the ribbon is quite low, often down in the 10 Hz range. There is often some internal adjustment, sometimes mechanical, sometimes electrical, sometimes both, to get the sound that the designer is after when the transformer is terminated somewhere in the ballpark of what he wants.
Remember that in a ribbon mic, you have a step-up transformer, which, for the preamp load impeance being reflected back to the ribbon, it's a step-down, and the impedance ratio is the square of the voltage ratio. So, just for an example, if it's a 10:1 transformer, a 1000 ohm preamp input impedance looks like 10 ohms across the ribbon. With active electronics between the ribbon and the connector, the ribbon always sees the same electrical load, so regardless of what happens on the other end of the mic cable, the ribbon always behaves as designed.
You probably are making a small mountain out of a small mole hill. While in any given situation, you can change the sound of any dynamic mic by varying its load, if you get used to what your mic sounds like with your favorite preamp, you'll lean to work like that. The ribbon motor doesn't generate a lot of voltage, so saving as much of that as you can is a good idea, but have some faith in modern electronics. Most modern preamps have about 60 dB of gain and at times, you'll need more than that from a "barefoot" mic if you want to get a decent record level. You'll end up with a better signal-to-noise ratio by putting a specially designed low noise amplifer ahead of the preamp to give you 20 dB or so of quiet gain that's also optimized for the mic. That's what you get with an active ribbon mic or an outboard pre-preamp like the Cloudlifter. But you don't want to put just any old amplifier there. By the way, somebody makes an active ribbon with a switch to bypass the internal electronics if you don't need the gain.
If you need a rule of thumb, I think it's reasonable to expect that the sound of a mic won't change very much, either in level or frequency response, when it's loaded with 3k ohms or higher. You might notice the effect of a 1k load with some mics, and not others. And for what it's worth, many years ago I reviewed a CAD Trion dual ribbon mic and it was so floppy that, while it didn't do the high end any good, I liked it better when loaded with 300 ohms when used on an acoustic guitar.