How do you make engineers automatically assist a building/experimental when they're built?

Subject line is essentially the question. Thanks!

Set a patrol route for your engineers and build an experimental near the patroling engineers.
They will automatically assist when your mass and ecostorage is over 80%

Gotcha. Looks like they won't help with building/ACU upgrades?

That's right, they only reclaim repair or assist build.
They don't help on mex/factory upgrades or ACU/SACU enhancements.

Trap engineers inside a walled out area and make them assist ground and they will kinda act like an engineering station

ZLO, how can you dare to say that ?
Taking engineers as hostage can't be the solution!

Oh also assist on ground make units lose collision, and that means you can pack all hundreds of units just in a few "squares" of walls