Same issue here.
I did use the suggestion link though, even though most fields don't apply. (I'll post an update here if I ever get an answer)
One thing that occured to me while I was describing the problem is that an engine that you make yourself but is not publicly available isn't really relevant as a tool in the same sense as, say, Unity.
So, while I do agree we lack the means to inform about home made engines, I'm not sure if the Engine & Tools metadata is the correct place to include this new tag. It would make more sense to include a new category to specify details about projects themselves.