Everybody's been there... you finish welding your blueprint, either manually or through some automated contraption that does it "automatically" and somewhere in there is a conveyor junction or tube that didn't get completed...
Tearing the ship apart and using "Show on HUD" for the damn block is a huge pain.
Having run into this, recently, on a blueprint of a mega-large ship, I did the following:
Grabbed the blueprint and wrote a program in python that removed all of the "non-functional" blocks (i.e. armor, catwalks, lights, etc.). I then projected THAT blueprint, welded all the crap up, and then replaced the original blueprint on the projector and welded up the "wrapping" of the ship... Then I got to thinking...
Couple of ways we can go about this:
1) Have a checkbox on the projector "Show only functional blocks"
2) Even better, have a list of all block types in the current blueprint, and allow the player to turn blocks on and off as they desire (because, for example, while Jump Drives, Batteries, Ions/Atmo Thrusters and Gyros are definitely "functional" blocks, they don't need any conveyoring, so having them in my "make sure all my crap is connected properly" version of the blueprint is just annoying)
Attached I added screen shots of the various stages that the BP went through for our construction.
Interestingly, and I hope you don't "fix" it, after removing all the non-functional blocks, if you paste the blueprint in creative, even though there are MANY blocks that aren't connected to absolutely anything (they literally appear to be just floating in space, like the antenna in the "second pass" screenshot), you can actually FLY the ship as if the blueprint were complete. The only difference is that the ship "handles" differently, because it weighs less (a LOT less in the case of this ship which uses, primarily, heavy armor)
If anyone is interested, I'm currently working on a version of the python script that's more "interactive" (as opposed to the original where I just hardcoded all the blocks I wanted removed) and would be happy to share.