Seaplane or
observer doors would either be Minor Alteration in my book, of course it's dependent upon how you hinge the top for the seaplane style. So long as you do no welding, it'd be a Minor Alt.... I've done several different varieties up here, on various airplanes and have used piano hinges or lugs similar to Maule hinge lugs, used gas charged shocks to hold open, or latched similar to Supercub system... Again, this is totally none structural change, and the Maule is a simple one to do with existing upper door cutout structure on airframe side. That's my take, and never had any issues with FSDO Inspectors, just need to properly document in logbook...
I think you could very easily win the argument with Inspector with definition of Minor / Major requirements no matter where you're at? We have that problem to some extent up here as well with varying opinions from inspectors, but bottom line is there's a very broad definition with all kinds of wiggle room that they are required to abide by, like it or not.... Sometimes necessary to point out that definition to them, and suggest they get second opinion from engineer type? See FAA Definition FAR 1 and you tell me?
Major alteration means a repair:
(1) That, if improperly done, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics,
or other qualities affecting airworthiness; or
(2) That is not done according to accepted practices or cannot be done by elementary operations.
Minor alteration means an alteration other than a major alteration.
