In v.12.5, an enhanced conflict refiner has been introduced which is able to report conflicts in CP models.
Concequently, the documentation has been updated -- e.g., the hint that the oplrun's -conflict flag only works with CPLEX models has been removed --, but so has the hint that -relax is only available for CPLEX models.
When I try to use the relaxation feature in Optimization Studio, I get an "EXCEPTION_ACCESS_VIOLATION" error (from the script that tries to access relaxationIterator from IloOplModel), and (from command line) oplrun only reports that CP solver wasn't able to find a feasible solution.
Long story short... no relaxation for CP models in v.12.6?
Edit: @AlexFleischer pointed out that
"...when you do oplrun -h you get
oplrun [options] model-file [data-file ...]
oplrun [options] -p project-dir [run-configuration ...]
-h this help message
-version print version information
-w show warnings
-relax calculate relaxations needed to get
a feasible model, CPLEX only
-conflict calculate a conflict for
an infeasible model"
...which basically answers my question [although, IMHO, it wouldn't hurt to have this piece of information placed (a bit more visible) in the docs].
So, the follow-up question is: is there a recommended (systematic) way to simulate "relax" using the information the conflict refiner provides?