How can I change a tunable parameter with a Matlab function when using Simulink Coder?

We have a Simulink model that is compiled and run on a dSPACE box. We need to be able to

1. change the value of a parameter with dSPACE ControlDesk and
2. change the value of a parameter with a Matlab function inside the model

1. is possible with a Constant block, just by entering a number. You can change the value of the Constant block in dSPACE. It is also possible to enter a variable name like FOO in the Constant block and change that value FOO as a tunable parameter. 2. has not been possible so far. Things like 'set_param' do not seem to work, because everything is compiled with Simulink Coder.