Like Mink, I've gotten DCL clearances from IAH, TEB, IAD, SAT, MSY and some others because in Item 18 of the flight plan, I have J3J4 and "1FANS2PDC" for DAT. Here's a link to how the whole thing works from Harris Corp.
https://www.google.com/url?sa=t&rct=...q97SJQ&cad=rja
Note that you'll get your oceanic clearance not from CPDLC, but from the same system you currently use to get ATIS. You'll be able to log on to CPDLC once you enter Canadain domestic airspace. As you coast out, it should automatically transition to Gander then to Shanwick or Santa Maria.
Enjoy.