Hmm. No time to think about the implementation, but it seems like it should be relatively easy to built that assembly as a set of curves describing the centerline of each tube. Give each tube thickness a different color, then run a script that sweeps a circle of that size along each curve. Do a few boolean operations afterword and you should have clean joints — possibly with some caveats when you're dealing with multiple pipes joining in a single spot. Keep the original curves around and if the frame geometry changes you can regenerate the pipe version.