Insert voltage-controlled current sourses in series with PMOS and NMOS transistors of output inverter. Complexity will depend on specs. Do you have any specs on linearity and tolerance of this controlled slew rate?
I have an Idea, but maybe is not the best.
Since slewrate limits the slope of the signal, you can use a diferentiator, then a simple variable voltage limiter, and the an integrator.
Slew rate is proportional to the driving current, so first build a linear current source, I think you can use some negative feedback to design, then use the current source to control you output current.