It seems it's possible to buffer basic casts. An easy way to try it out is using the training dummy in the Crossed Wands area. When you get to 4 basic casts in a row, just tap your basic cast key 4x inside of about 1 second, and you'll automatically fire off all 4 casts in succession.

This appears to work just like any standard buffering, queueing them for the first available frame. Not a big time save, but added up over a run, it can be, especially if you factor in the extra 1-1.5 seconds you have to read the battlefield or react while the queue is dumping.

It also means a bit less fatigue knowing it may be completely pointless to just spam the key to try to press as early as possible after a cast finishes.