I have faced the same issue, my workaround was to define a process variable and set the desired constant value to that process variable.
For example if you're using the eclipse editor you can define a process variable YourConstantVar in the bpmn and then assign a value to it in "On Entry Actions" of the subprocess:
Then map the subprocess input to YourConstantVar and it shall work.
Thanks for reply. I used the same method.