Perhaps this is official policy, but in practice I've had no trouble redeeming Delta certs (that I've either received as a PM benefit or as compensation) for flights on partner airlines, although booked through Delta and Delta coded.

In fact, as an additional data point, if you ever find yourself in possession of an AFKL voucher, you can redeem that with DL too, though you'll have to call in and the agent will need to contact the JV or GTS desk to apply it after building your itinerary.