I'd like to find something like this....

Accessory Mounting Block, Tapped 1/4-20 w/Knob | | Berkey System (http://www.berkeysystem.com/store/blocks/accessory-mounting-block-tapped-holes-wknob/)

with multiple mounting holes, that splits in half so it can be put on a tube that has permanent, larger ends on it.

Anything come to mind?

I don't know much about this kind of rigging, but I happened to come across this, if it helps:
Swiss Rod 15mm Rail (2pc) by PNC | Photography and Cinema - Store (http://www.photographyandcinema.com/products/swiss-rod-15mm-rail-2pc)

Sounds like you're looking for an articulating arm clamp. They come in different configurations. The photo on the right shows one with a 3/8 and 1/4 hole. Or if something a little smaller, the other clamp has two 1/4 holes and is designed for 15mm rods (available at CoolLCD).

B&H have a huge range of 15mm mounting accessories. At least 14 pages from what I can see

Rod Brackets & Adapters| B&H Photo Video (http://www.bhphotovideo.com/c/buy/Rod-Brackets-Adapters/ci/12018/N/4184435833)