No time to try myself right now, but ... symlinks?
Thanks for the suggestion!
poudriere.conf has ZPOOL=tank and ZROOTFS=/iocage/jails/poudriere/data.
I ended up with symlinks in the jail at /usr/local/poudriere -> /mnt/usr/local/poudriere and /mnt/mnt -> /mnt.
That gets poudriere to construct it's jails correctly.
Unfortunately, after that I get "file name too long" errors from mount. Possibly solved in TN12, but I already backed away from that until at least U1.
UPDATE: Got it to work completely by shortening to /mnt/p and just 'f' for the jail name. Hopefully TN12 will let me use sane names again when it's working for more use cases.