We already have the solver rosenbrock_posdef_h211b_qssa in the
user_contributed directory. Unfortunately, it has not been maintained
recently, and it hasn't worked with KPP3. Together with colleagues from
Jülich, I have now upgraded it so that it works out-of-the-box. We are
also writing a manuscript about the solver which should be ready soon.
The new code is in the branch
https://github.com/KineticPreProcessor/KPP/tree/feature/h211b. It
contains the new solver file int/rosenbrock_h211b_qssa.f90. You can
also find a simple test case in examples/mcm_h211b.

I hope that the code will be ready for a pull request soon. In the
meantime, please let me know if you have any comments or suggestions...