Which rules did you want to combine? I think you could fit the powers checks, madness/horror/fear checks and Masque monster powers in. The game is fantasy version of North America that takes certain liberties with history (like some of the Vikings that came over survived to colonize certain parts of the mainland instead of leaving). The magic level resembles more to the level of D&D (perhaps slightly less as arcane magic is generally looked down upon by the Uropan powers) than Masque of the Red Death. The setting takes place in early colonial days, 1666 IIRC, but it has some of the trappings of later periods (like prestige classes based on Johnny Appleseed as well as Paul Bundy in addition to a headless horseman monster). It uses several of the D&D core classes (wizard, sorcerer, cleric bard, and druid) as well as putting some new classes (agent, natural philosopher, raider, rake, scout, soldier, and witch). They are all generally much more powerful than the classes in Masque of the Red Death.
Also, if you are curious, the book is linked to the African equivalent: Nyambe, which has its own book.
Edit: here's some reviews of Northern Crown:
http://www.enworld.org/reviews.php?do=r ... id=2392922
http://www.enworld.org/reviews.php?do=r ... id=2397801
I think the main points of contention between Gothic Earth and Northern Crown would bet time period and level of magic/fantasy. It would probably be very difficult to try to mix the two class systems together. However, I think you could mix the essential essence of Gothic Earth (ie. human dominant and the existence of the Red Death as a malignant force) with Northern Crown.