Something like that can already be approached with a Script and Smart Objects (without preview albeit).
But such a feature with preview and applicable non-destructively would be welcome (if at all feasible programming-wise).
Though I don’t quite get the two-spline-thing; I think an option for defining a start- and end-size or size-decrease-rate might also do.
Anyway could you post a mock-up of the effect you want?
To make sure whether you want a »one sided motion blur«.
Something like here:
But I think having one- and two-spline motion blur would be enough.
Each spline would need separate blur/speed settings.
Doesn't have to be non-destructive, preview would be nice, though.
That does look nice!
I hadn’t gotten how the multiple splices are to be employed before.