How about the following?

-rename DTYPE_ESDI to DTYPE_ATA in sys/disklabel, keeping
 the numerical value (5)
-use DTYPE_ATA as appropriate in wd drivers and bootblocks
-add a new type number (would be 16), called DTYPE_ESDI,
 and use it in ed drivers (and bootblocks)

This way existing disklabels would be interpreted correctly.
The backside is that this would affect disklabel compatibility
to other Unix versions, but afatct there is not much standardsation
anyway.

best regards
Matthias