Analog RMS converter have a problem if the signal gets too small (e.g. below some 1% of full scale). The problem starts with the rectification part detecting the sign - first it gets slower (reducing the BW at lower amplitude) and finally fails. Some version also get a rather sluggish response at low levels.
Things are better (or at least can be), with usually expensive (though it does not have to) DMMs that use digital methods for RMS. So the Keysight 34461 could actually work well even below 1 mV, though the specs do no include this. The advantage here is that there is no need for a separate sign detection. So the BW can be constant and no magic limit where things break down. Residual noise compensation is also easier in software than in analog HW.
100 µV are still 1 % of the 10 mV range of the cheap meter. So chances are it could still work - at least for low frequency. However such a test can vary from unit to unit - some may work better than others.
Turning off RMS is easy, just use the DC range
. Of cause this does not solve the problem unless the meter is really fast and can do the math (like man DSOs can).