Building utilities on a wall corner

I struggled with this for a long time, and in the end, no, there's no other way to force them, because they cannot physically get to where they need to get. For now, destroy the wall pieces that you need, then rebuild them.

Consider next time, figuring out where your toilets/plumbing items will be, and lay the pipes before building any of the walls. (The planning tool is extremely useful for this) and it will avoid the situation entirely.

It is usually more efficient to lay the foundation and entry doors, build the utilities, furniture and special floors first, and then when all of them are finished build any interior walls. This has the advantage that the workers can move freely across the building without having to move around walls (or even worse: through prison doors they can't open without the help of a guard). That way they can finish the construction project much faster.

The downside is that you can not use the clone-tool that excessively because you can not use it selectively to not clone walls.

This was identified as a bug (#0005061) and is fixed by updating to Update 7.