Rigging, yes. Violating a size limit, no. FP plus the small HG-11 handgrip with the Lanparte T5C SSD holder screwed into the side of the handgrip is still tiny with a slightly fatter grip. With the this set-up, it's about the same size as the Fuji X-S10 (depending on your lens) but shorter because of the Fuji's hump on top.
This is what I found the most annoying because Sigma has good stabilization in their EF 24-70mm (and others) but left it out of their L mount version.
As for the AF, I've shot a bit with the S5, also. They're both fine with acquiring initial focus as @MrSMW suggests but I wouldn't leave either of them in continuous for a single-cam talking head interview of any length.