What is the usual dress code for academic workshops?
Academia casual, as you put it, is perfectly fine.

Basically, imagine yourself doing a TA in front of students. I would say, a similar kind of attire would suffice.

I've seen people doing an academic presentation in a T-shirt and slippers, and that was a damn fine presentation, too! Even if you'd rather want to wear some more formal footware than slippers and some jeans and not shorts, I think that my point is clear:


No beachware.
No three-piece suit.


I've worn a shirt and formal trousers to multiple conferences without feeling overdressed, but that's probably the most formal thing you'd wear if you are not the conference chair.