EDIT I use the photosounder spline EQ which makes a great tilt EQ even in the freeware version with two control points. Shove one to the left but leave it at 0 dB and one to the right and lower it by the desired amount, done. Or vice versa. http://photosounder.com/splineeq/
However it would be great to know which hardware units are designed as tilt EQs (besides the well known Tonelux) and whether anybody has sampled one for Nebula.
I think standard predelay which is somenthing forced by init program in our current nebula release doesn't allow that, we have only 1.5 ms. Anyway try to sample your spline curve in nat (even offline), using a bypass session (or a preamp tuned to a single sample using number parameter in expert page)