The early San Ans used a main pivot that ran on a Delrin bushing. The clever thing is that they machined a helicoil thread on the outside of the pivot which lets you pump grease through regularly without any disassembly. You'll need a grease gun with a long nozzle. Now at the moment I can't remember which side you pump the grease into but you'll soon see as there is a hole in the centre of the recess. Take the plastic cover off the main pivot - it's hollow inside until about half way through. Push the grease gun in and locate it in the recess then pump the grease in - this purges grease between the pivot and the bushing and you will see the old grease forced out past the pivot o-rings between the swingarm and frame.

I'm not convinced it's the main pivot creaking though, could be anything from bottom bracket, saddle rails, sunframe mounting points etc etc as the monocoque amplifies any minute creak or groan!
Just work your way through cleaning and greasing all the possible culprits.

Will post again once I've double checked which side of the pivot has the grease port.

Gus