By creating a in game currency, it'll probably be easier to work with the EULA. Also, it does save a butt load of work trying to implement Bitcoin into the server. The virtual currency could operate the exact same, and is probably a more secure implementation, since you aren't storing Bitcoin on the server. Instead, users would earn the virtual currency, and then request a withdrawal at x time.
The question is whether there is a guaranteed exchange between the in game currency and the external cryptocurrency. If there isn't, it might be hard to convince people to still deposit real, externally usable cryptocurrencies. If there is, the in game currency might be regarded as a unit of account for the external cryptocurrency, and thereby possibly a collision with the EULA.