On two systems running 11-CURRENT, I configured encrypted swap (both manually through shell and through the installer front end). Both dumps I switched on the setting for dumping to dev, which added:
to /etc/rc.conf as expected.
However, when panics occur and dropped into KDB, 'dump' would not work:
Cannot dump: no dump device specified
I ended up going around in circles for a while, until eventually I was explained it worked by dumping to swap, and I volunteered that my swap was an eli drive.
So since then, I reconfigured my system to use a swap drive without eli and now dump works.
I think KDB should give a more helpful message for encrypted swaps. KDB doesn't support dumping to encrypted swap. This confused me for quite a while, since there isn't much in handbooks / forums / lists about this shortcoming.