I've done a 40 minute turnaround at BUD on an LH M&M mileage run and I had the same concerns. Here's what I learned... we were brought to an outside location, so had to take a bus to the terminal. Just inside where they drop you off there is a transfers desk where the Agent happily printed of my return boarding passes BUD-MUC, MUC-FRA even though I was flying on electronic ticket. Then I had to go through immigration, but of course as I had been 5 minutes getting my boarding passes there was no Q, straight up the stairs to departures, through security and airside again before they even called my outbound flight. Feel free to PM if there's anything else you need to know, and enjoy the trip!