Maybe your dealers are reluctant because they do not have a reliable machine shop to send them too. We have sent out quite a good amount of cylinder heads to have the valve seals replaced, but only in E65s, E60s, and E63s. Havent seen any E53s that needed it yet (+1 for us!) but I can tell you that every single oil burning problem has been fixed by replacing the valve seals. They also
clean the entire head and install new retainers. They look brand new when we get them back and have had no problems.