I realise this is an old thread, but since I've recently had both seat bases apart (to rub down and respray the base units), I thought I might add my ha'pennorth. I've never had a seat seize on me, but in the process of tearing everything down I identified what I think could be the problem.
While I had everything apart I had a good look at the mechanism. It's pretty simple and I'm fairly sure that bit that gives trouble is where the metal column under the seat slots into the nylon (?) sleeve in the base unit. On my seats both sleeve and column were covered with old, dried up - and very sticky - grease. My seat sliders weren't working too well either, and this turned out to be the same issie.
The second part of the mechanism, which is a two-piece helical insert that allows the seat to drop in height as it swivels to face rearward, is well protected and on mine still well greased and smooth, so I left alone.
Wiping off the dry, sticky residue of old grease and applying a thin film of fresh grease - to both swivel base column and seat runners - has made a lot of difference.
The message, I think, is clear: separate the base from the swivels, give it a good clean and put some grease on BEFORE it seizes. After 20 years it's not really surprising they need a bit of lube and, if it's not had any attention in that time it's not so much a question of whether they seize as when...