Step 2: Say the unholy incantation:
parent_type = /datum
Step 4: PROFIT!
I keep expecting SOMETHING to break, but some very basic tests show this works to make vars and procs defined on /datum exist on your client objects, as well as make client's pass an istype(client, /datum) simplifying some code, with no observed issues so far.
Update: /tg/station has been running on this setup for a while now, and no issues or oddities reported.