You have a point... but I can already do that now by giving them gifts. I can see what can be built in vassalls' provinces, I can't see that in other realms, so players are obviously supposed to fund that stuff. It's just a painful click orgy in a big realm having to it by gifting.
This might be an idea for CK2. To do it in CK you'd have to hack the executable. It's not an easy mod.
Building stuff in a vassal's province should not be painless. At a minimum there should be a loyalty hit. You're meddling in the internal affairs of a vassal, and they ain't supposed to like that. They should also be able to refuse if you have Feudal Contract law, particularly for improvements that aren't just free money.
I know I'd be pretty pissed if I was scrimping and saving to increase my piety and my jackass liege built Moneylenders without asking. Or I was several thousand in debt and my liege imposed a Courthouse on me.
Even roads should be rejected sometimes. A committed rebel is not going to let the Royal workcrews build a highway right up to his castle.
Nick