I am _pretty sure_ that when you use the nopass option with the full path specified, it requires that you use the exact string (i.e. the full path) in the config file.
So with your current config ...
Code:
doas /sbin/mount /some/device /some/directory
... should work without a password.
If you change the config file to just "mount" and "umount" I think that it will work like you expect it to, BUT that could be less secure, I remember reading somewhere in the man pages (can't find it now) that anyone could make a script called "mount" or "unmount" and run it with the nopass option, if your doas.conf is configured without the full path to the executable.