If you can elaborate on what you mean by centralized post or how you’d think you’d best learn about amas I’d be happy to try it.

Another thing we’re thinking of doing is having AMAs on one fixed time/day per week

A set time would be nice.

Maybe a sticky post? But typing it out, i don't like it that much actually... It would imply some moderation to decide what to sticky. Which is not really SN's leitmotiv...

Maybe just announce them over Twitter (unless you already do), couple of days ahead of time.

We do announce them on Twitter! Usually well in advance from the stacker_news acccount.