Check the bespoke driver in the Sage's sources (drivers/misc/ntx_cilix.c), that'll probably answer most of your questions.
Code:
cilix.c - driver for i2c through uart bridge
I would strongly advise *against* buying one, even for testing purposes, because I can't even really say that it's working as intended: it behaves... very, very, very badly; so the net effect is pretty insignificant; making it barely more useful than a very expensive cover
.