My 15,000m at under 100bpm was at 24-25spm and an average pace of 2:30/500m, DF 125 (always)
I can hold 2:20 at 24spm for 15,000m and keep my HR below 110 (this tends to be whilst watching TV episodes).
Likewise 2:15-2:10 for 15,000m at 24spm and keep it below 125bpm (waiting for people at the club).
24spm seems to be my 'go-to' stroke rate regardless of pace (unless it is a time trial when it gets pushed to 28 or 30+).
My weekend warmup while waiting for everyone else is also mainly done at 24spm and starts at 2:30/500m, then drops 5 seconds off the pace every 500m, so 2:25, then 2:20, and aiming to stay at or near 24spm, although when I hit 2:00 and 1:55 it usually drifts up closer to 26 or 27, that's 3,000m, I then drop it back down again to around 2:15 or 2:20 as a 'cool down' then continue at this until everyone else has arrived and stop at either 5,000 or 6,000m depending on when they all arrive.
As a group we do our UT2 sessions at 18, 20 or 22spm, which is lower than I like, and in time with each other, but have HR limits, mine is usually restricted to 135-140bpm, we tend to get a bit competitive over distance rowed as most of the sessions are time based so as not to be too demoralising for those who don't 'live on an erg'.
We had an ErgRace 'UT2 session' the other week that was most definitely a race with several people going way over what they were supposed to from a heart rate point of view.
As Terry mentions a UT2 'workout' is not the same as a 'recovery' session, In a recovery session, I may/may not break a sweat depending on the duration of the piece. After a proper UT2 workout, I will be dripping just as much as at the end of a UT1 or AT piece.