I usually enter the MUC non schengen (H Gates) SEN lounge for a quick shower and breakfast on arrival.

The procedure is simple:

On arrival, head for the H gates: you will have to clear security.
No one asks you for your onward BP at security for the H gates. Makes sense, as some may be connecting to OAL and may not have a BP for the onward flight. Sometimes the customs spotters ask you what you are up to, but 'I'm heading to the SEN lounge for a shower' is good enough.
In the lounge just show your BP and the SEN card. Never had any issues, even after the rule changes.