The main problem with the HQ is the upper part of the carriage is built into the machine, so there is nothing to put another machine on.
I've never seen a carriage that will fit the HQ frame. . .
In a real pinch, I'd put the quilt on the frame to hand baste/pin baste and then take it off and FMQ it with an easy pattern on my Juki.
This got me thinking, and my DH said "didn't you get a second carriage when you got the robotics?" so we got into the closet and found the box with the non-robotics carriage. DH is going to fix me an upper slider and then I'll have a carriage I can put a different machine on if I ever need to.