Before the mesh becomes cloth, drop down a point wrangle. Add a float value to the interface. Place your keyframes there.
f@my_attrib_name = chf("animate_me");
After the wrangle, drop down a NULL named OUT_ATTRIBS.
Dive into the solver to configure the vellumconstraintproperties. On the inputs tab change one of the non-used inputs to fetch from sop, OUT_ATTRIBS.
Then it's just a vex transfer of the animated value to the attribute you want to drive.
float my_animated_value = point(1,"my_attrib_name",@ptnum);
stiffness = my_animated_value;
Don't forget to turn on the check box for the Stiffness value so the code can overwrite it.