Is it legal to put an Extra Capacity Base Pad in a post ban Pistol mag?

AFAIK Not if it will take the mag capacity over 10 rds.

I would have to say nope can’t do it if you can take more than 10 rounds.
However, if you could prove in a federal court that a magazine extension is a separate ammunition-feeding device then yes you could have more than 10 rounds in that magazine.  In fact if that were true then you could chine link a series of 10 round extensions together to give you any amount you wanted.  

An example of such a device is a magazine clip that holds several magazines together, so essentially that device could hold 3 10 round ammunition-feeding devices.  Although in CA I don’t think you can have any more than one ammunition-feeding device connected to a single weapon at any given time.

...or you could just carry another pistol and save on court costd.