What you could do is set the number of devices in the array to 3 so
they it always appears to be degraded, then rotate your backup drives
through the array.  The number of dirty bits in the bitmap will
steadily grow and so resyncs will take longer.  Once it crosses some
threshold you set the array back to having 2 devices to that it looks
non-degraded and clean the bitmap.  Then each device will need a full
resync after which you will get away with partial resyncs for a while.

I don't undertand why clearing the bitmap causes a rebuild of
all devices. I think I have a conceptual misunderstanding.  Consider
a RAID-1 and three physical disks involved, A,B,C

1) A and B are in the RAID, everything is synced
2) Create a bitmap on the array
3) Fail + remove B
4) Hot add C, wait for C to sync
5) Fail + remove C
6) Hot add B, wait for B to resync
7) Goto step 3

I understand that after a while we might want to clean the bitmap
and that would trigger a full resync for drives B and C. I don't
understand why it would ever cause a resync for drive A.