Studying koans is not a very good way of gaining understanding of something like Buddha Nature; they're designed to create confusion. I wasn't in a Zen tradition, and I'm no longer a Buddhist; I appreciate your respect, but I'm not about to start trying to figure out koans. [Edit: aren't you supposed to have a master, when you're working with koans?]
Note that I'm not declaring Buddha Nature teachings to be heretical; in general, I think the really deep divisions between the various Buddhist philosophical positions were (are?) more political than anything else, and I've never considered them important.
I've studied both the Uttaratantra and the Uttaratantrashastra, under the direction of a western academic who specialized in the subject (and also happens to be an ordained Buddhist nun). I've also done meditation practices designed to deepen understanding of Yogacara-style thought. I think I have a reasonably good understanding of the subject, for a lazy, lay practitioner.
The accusation of heresy is because it's very easy to misinterpret Tathagatagarbha teachings as monism.