The answer? Tubing clamps. They are easy to make, and quite versatile. On the Barracuda low racer, I am using them to hold:  the front boom, the front idler, the seat, the intermediate idler, the jackshaft, the remote steering arm, and the remote steer tube itself. Why not just braise these items directly to the frame? Because, unless your bike is designed perfectly, they will probably have to move and you may want to do some redesign later. It's much easier to just unclamp a bracket.

To build one of these, measure the Outside Diameter (OD) of the tube you want to clamp to. Find another tube with the same Inside Diameter (ID). Cut off about a 3/4" section. Get some 1/4x20 thread 1" long nuts (available at hardware stores). File one side to match the contour of the tube. Braise a long nut on each side of the tube.

You can then braise on any brackets, attachment points, nuts, or tubing to the clamp.

After you have finished braising the nuts and whatever on, cut the clamp in half. Using your 1/4x20 tap, clean the threads out on one side, and drill out a clearance hole for your 1/4x20 bolts on the other. I use socket head 1/4x20 bolts all over. They are strong, provide good clearance, and look cool. You will then need to file the bracket so that there is a small gap between the cut ends of the nut when you bolt the bracket onto the tube. If there is no gap, the clamp will not be tight enough.

Some examples:

This type of clamp can be used to hold a seat onto a round tube. It also allows for the seat to be slid forward and back.
This type of clamp can be used to clamp tubing in seat stays. It is also the basis of a boom tube clamp. You will want to line up at least two clamp bolts along the split in the tubing to attach a boom.

Here is a (bad) shot of the basic two piece tubing clamp.