So here's an idea. Why not use modifier + mouse wheel for this?
Example:
1. Pick what you want to build through the build menu as usual.
2. Shift + mouse wheel to determine elevation.
3. Click or shift-click to build as many objects as you want on that elevation.
It's (relatively) easy to discover while unlikely to be discovered by accident, which was the problem with the original implementation. It doesn't require any complicated keybindings.