I understand your needs, but unfortunately it's not possible.
There are parameters which depend of the Generator state, but even the fixed ones (as XSplineLength) may cause problems: what if a spline contains several sub-splines ? Should we use the first one, or what ?
But that is not the main problem: Generators are evaluated in order, and their splines (and other data) dosn't exist until they are processed.
For example, splines suffers multiple transformations before being used: they are clipped, filtered by ID, divided by evenly, divided by markers, conformed to surface, converted to polyline, and much more.
These steps are done before Generator is processed, and their data and parameters are deleted once generator is done. We should rewrite a lot of code to make it persistent between generators.