It would be easy enough to take the bases off the stands but there would be issues trying to thread the plated stands and cutting threads would also weaken the stands. If I was doing this I would mount pipe flanges on the riser and then thread a 6-8 inch piece of brass stock that was turned to fit inside the stand. You could insert this part into each stand and secure with a roll pin.
That's along the same lines I was thinking. Of course the one piece of equipment I don't have is a lathe. At least for a prototype I was planning to take up the slop with good old JB Weld. I do have a pipe threader so that's a pretty easy one.
The rack was another option and I did use one for a while quite a few years back. It could have been the hardware but I could never quite get things were I wanted them. That or I was just too inflexible to adjust myself!
As far as the riser, it does go to every gig with few exceptions. Right now we've got an old plywood setup another drummer had retired. I do have 2 so I plan to keep the second set of stands functional for the odd gig without the riser.
I did think about placement as well. My setup is pretty well established. I've marked all the stand positions on the carpet already. If I can transfer those to the new riser it shoudn't be bad but relocating after the fact will be a process.
