Server failover

What's the best way to get chronyd to promptly stop using a server, from
the server side? So the best option I have right now is to restart with an
alternate config file with "local stratum 15" in place of the refclock line.

Background:

We use redundant time servers in each of our datacenters, and we'd like to
actively switch away from time servers before their scheduled reboots,
because the estimated error that accumulates on our clients while waiting
to notice that a server has gone away can be too high for our standards.

One way to deal with this would be to run "chronyc offline" and "chronyc
online" on every client, but we have a lot of clients and they're already
polling the server, so I'd hoped we had a way to do it from the server side
instead.

It looks like there's no way to affect our use of the PTP refclock with
chronyc or otherwise change our stratum.