How to implement Cascading Crits on attack rolls (Pathfinder/DnD 3.5/5e/etc.)

Is there a simple way to add the ability for crits to cascade? For example, if I roll a nat20 to hit and then nat20 to confirm, is there an easy way to set it up to confirm another crit after that?

I don't think so.  Normally all rolls are made at the time the macro is executed.  Often, an additional roll result for crit confirmation is shown only if a crit threat has been rolled.  What is included in the final attack roll output is determined by the sheet's roll template code. PF for example only requires a single crit confirm roll so no additional code is included for cascading crit confirmation rolls.  I suppose you could simply include some additional attack rolls in the attacks Description/Notes which could be ignored unless you are cascading crits. ;-)

Is there something I could add to the macro field (maybe a looping clause of some kind along the same lines as the initial "on crit threat, roll crit confirm") so it could auto-execute on roll?

The macro language doesn't support any sort of flow control constructs like looping or if/then statements.  What you're asking for could likely only be accomplished automatically via an API script, which requires the game creator to be an active Pro subscriber.