Hi there,
Sure no Problem, So I went and verified It is supported in the latest BSP’s but it wasn’t on the initial release (may have been a beta)
Initially it was the debug, and LP stuff was flaky but they got it sorted in LONG list of BSP’s (must be a record, espressif)
Here is a comparison chart from the Technical reference guide Linked here,
It’s my opinion the juice isn’t worth the squeeze the small buffers for TX and RX for example, I can see why its not famous… LOL
@grobasoz is using it albeit NOT with the first BSP I’m betting 
so I’ll yield to his more current examples.
connecting to the bottom of the the xiao is also a task unless you got one of theses 
or just like to solder 
HTH
GL 
PJ 
We have used the Jtag pins on the bottom of the S3 for GPIO , no problem, just fyi Good stuff and have fun. 