To see if they have explored your return spring theory or something else?
Either way, why hesitate? I'm sure they will be open to suggestions, theories, guesses, practical examples of work done to arrive at a complete and satisfactory fix or just a very simple, 'Have you thought about?' type of enquiry.
There are three initial potential points of contact:
1. Through BMW's dealership network. Though that would rely on them pushing your observations / theory / practical and reliable fix, up the chain.
2. BMW's UK headquarters at Bracknell, again relying on them to push your correspondence forward.
3. BMW's headquarters in Munich
Contact details for all three available via a Google search.
There again, many people believe that BMW closely monitor the pages of UKGSer, including no doubt the 1200 Technical section, probably as a valuable adjunct to BMW's own and costly R&D function. So, it's quite possible that they are already aware of this thread and are on the case as we speak. However, English may not be their first language and / or they may well have become distracted by requests for advice on jackets and the legal aspects of company liquidation; so best to get your letter into the post. Of course it's worth it.