Back to the original theme - IIRC the Extended Warrranty might require you to have the servicing done at a Motorrad Dealer. This is allowed under Block Exemption if the EW is something that you pay for (because you are entering a contract and the manufacturer usually makes it a condition).
There's no exclusion I can see in warranty book. There is a mention of service history in the 'Making a claim' section, but no mention of non BMW exclusions that I can see:
The service history of the insured motorcycle and supporting invoices will be required by the repairing retailer at the time of the insured warranty claim.They don't have to offer you warranty in the first place, of course, so if it's a bike they haven't seen go through a BMW dealer for, say, 5 years, they might say 'No thanks, we don't want your custom'.
But if you're already in warranty, then I can't see anything stopping you having it serviced by AN Other and still staying in warranty, other than the warranty may not be available to you the year after.
If AN Other is proven to have bodged something, then I suspect all bets are off at that point, though.