I'm a Swiss citizen who was born and raised in the US. I've only worked here five years, and am considering paying into my AHV account (and my wife's) retroactively.

Does anyone know if there is an absolute minimum contribution? Other relevant information would be greatly appreciated.

I believe you can only pay missed contributions within a 5 year time span.

The minimal contribution if you have no income is around 400 CHF/year. There is a 5 years limit for retroactive payments. Not sure about further conditions, best approach your cantonal AHV office.