TL;DR
Add an option to blueprints so they will rotate 4 ways, 2 ways, or not at all. Ditto for flipping, similar to how some blueprints already cannot be flipped due to entities like train stops.What ?
When editing a blueprint, near the grid options there would be one additional three-value option "Rotation" with options "90 degrees", "180 degrees", or "Disabled", and an additional boolean (checkbox) option "Allow Flip" checked by default (or "Forbid Flip" unchecked by default).These options might be enabled/disabled when the blueprint contains entities that affect the behavior in question, e.g. train stops would grey out "Allow Flip".