As for the set screw the way it works is this. The gimbal basically has a L cut in the bottom of it so when you tighten then screw down more surface area pinches around the post.
You'll notice in one photo with the post flipped the gimbal only goes about halfway up the post. Thats with the arm fully boomed up. When you put weight on the arm you can sneak it up more.
As for drilling a hole through the gimbal I can do that to secure a dbracket on it. Its just a matter of whether or not the dbracket will have clearance on the arm. That IIIa style jbracket looks good. I would just put an aircraft pin through it for peace of mind.
My arm is rated for 76lbs. I have not flown that much yet. But I've not noticed any flexing of the sled with 50lbs. And I'm sure the arm will handle the 76lbs. And the arm won't flex its bullet proof.
I like the backmounted vest Brian designed. It takes a bit at first to dial it on. Once its there its there. I've seen other users with it way bellow the hip pretty much on their butt. You can see Ramone on the behind the scenes of Good Eats
like that. I'm 6'4'' and have had mine just bellow my hip.
But after using the Ultra2 vest I fell in love with it and would trade my Backmounted for the new Ultra Vest or a Pro Vest in a heartbeat.