During the last 20 days, the pending_rewarded_vesting_steem value declined from 524K to 506K, ~900 per day. None of this seems to be documented (outside of code) - and I'm not sure of the exact mechanism, but if I'm surmising right, there would have been a corresponding slowdown in virtual_supply growth(?).

Interesting thought. The question is whether there is really a relation.
At the moment, I have also found no indication in the code that this has an influence on virtual_supply.

Unfortunately, I have not yet made any progress with the evaluation of my records. I won't be able to do so in the next few days either... we'll stay tuned :-)