I actually found limitations in the implementation of th MCPWM hardware which were not so clear to me from the documentation:

1. A roup cannot have 4 operators. 2 operators worked, but I tried 4 (for the two half bridges and the two enable inputs, and got the message "no free operators in group 0").
2. One operator can have up to 2 comparators.
3. One operator can have up to 2 generators.
4. A generator can only react on comparator events from itsown operator.

With these limitations I currently don't see a way to generate the 4 independent signals for half step mode. Feel free to use my test system on github as a base for your research.