There may be another way to approach this problem: Although artists cannot challenge his legal ownership of R&D rights, they could possibly sue him for loss of earnings. If Bulmer controls the R&D rights of work on which artists depend to earn a living, then perhaps he can be legally obliged to enable them to do so. If Bulger is preventing people from legitimately earning a living from their own work, he has a legal case to answer. I think it's worth pursuing.