Here are a couple of scripts that might help -
This one picks up the Z level of each selected curve and puts it on a separate layer according to Z height - the layer name is the Z level rounded to two decimal places. It was originally designed for topographic curves, but it should work.
LevelsToLayers.py (1.8 KB)
This one selects by Z level - you can give it a range of Z values to select or just one Z value and Enter to select just that level
SelectCrvsByZLevel.py (1.6 KB)
And this one that selects all curves on the same Z level as a selected curve
SelectCrvsSameZLevel.py (1.8 KB)