If the goal is to dismiss it manually in the first few seconds of the game then there is no need for a toggle. I can just make the decal disappear after a given time, say 30 seconds, while you're building your first factory.
Having a toggle is possible if I make a PR for FAF that on pressing some key combination it checks the map folder for the decal in question. If it is there, it creates it. If not, it ignores the keyboard combination and adds a message in the logs indicating that it doesn't exist. One issue here is that a typical decal would be visible for everyone. E.g., an opponent toggling it to annoy the enemy. Creating a decal for person A and not person B will cause a desync. There are UI decals, but I haven't worked with them before.
Generating a decal at startup is something @Sheikah could do, especially for map gen maps. The node network is not complicated and is done in under a second. Generating and inserting them for non-map gen maps is a bit of a stretch in my opinion.