Is there some generic rule for what happens to you if you eat the Heart, drink/bathe in the blood of a powerful supernatural creature?
The Transylvanian book mentions a hedge tradition whose members gain the Giant Blood virtue when they kill a giant.
Siegfried and the blood of Fafnir.
Some of the tales of Valravns, and Perytons.
While I expect that you would earn the enmity of House Bjornaer if you killed a Great Beast and ate its heart, would you gain any virtues doing so?
No general rules for that, no. If you gain anything at all from doing so, and what you might gain, is case by case.
There is no such rule, but it makes a perfect Quest for Initiations. For example, GoTF p.39, the 2nd Quest for the Path of Energy is "Conquer the Dragon of the Sauerland, and eat its heart."
It is clearly a non-hermetic ritual. Considering what was done to House Diedne being accused (amongst other things) of blood rituals - regardless whether it is true or false - any mages should be very careful while investigating such practice.
Alternatively, it kind easily be part of an Initiation script for a Warrior or Hunter Cult.
Definitely seems a possible quest for gaining the Imbued with Spirit of [Form] virtue.
Or the Mythic Blood virtue
But it could well be developed into a hedge tradition.
What kind of horrible Pagan would eat the heart of a supernatural beast or bathe in blood.
It is not like Christians eat the flesh and drink the blood of their Saviour.....
It's not weird when we do it, because transsubstantiation is literal.
Also, no one will convince me otherwise: the beasts of virtue of species that are traditionally eaten must be even more delicious. A boar or deer of virtue would be worthy of a king's feast.