An automatic glabel(8) device (but not a manual one) should append "/label@<LABEL>" to its underlying provider's physical path. Manually created glabel devices shouldn't do this, because they provide exactly the same data as the underlying device.
Turns out this isn't as important as I thought. Due to the way ZFS aligns its labels, the glabel automatic label will never affect ZFS. ZFS neither reads that portion of the disk, nor will shortening the disk's length by 1 sector affect the position of ZFS's labels. So zfsd is perfectly capable of using either the bare disk or the labeled disk, and zfs won't damage the label. Since zfsd works fine, I'm going to close the issue.