The crux is how much loss in spring damping/flex has occured due to the sagging. If it could be measured then matching the loss to a compensating extra could spring might work. Basically replace the lost effect. The difficulty is in working out the losses due to sagging so you can calculate the right spring wighting to replace it. Furthermore, how long would it solve the problem for?
The best solution is new springs or SSL.
Rog’s bump stops looks like a similar solution in some ways. As deflection increases the rubber is brought into play at extremes of deflection. Adjustment can potentially be made over time.
