Pledge existing property to increase equity?

No, if you need it for the financing, then she also wants to be entered in the land register. That might perhaps result in a 0.0x% better interest rate, but I don't know enough about lending practices for that. Maybe knows something about it (hope I chose the currently still functioning nickname...).

I also only know it that way, that the "non-buildable" plot is included in the financing with its own land charge. So to speak as a substitute for equity (from a risk perspective)

This should result in a lower interest rate.

However, it is only a substitute for equity, not real equity.
The financing amount and thus the monthly burden remain the same (high).