NO having other work done does not invalidate the registration. Trust me on that one. Now go and dig out an old photo of you holding the guitar way back when. Funny Martin will not accept a certified letter swearing you were the original owner, they don't accept that anymore. But if you can show them a photo like I did way back when with the guitar, then they say ok and will register it for you. I had bridge work done on my guitar prior to the registration and Martin still had their authorized repair place fix it and paid for it in full with the exception of the new fret job

unless the work you had done before totally messed up the guitar it will not affect your warranty ... I just went though it 13 months ago