I don’t think there is an easy way. The 7 hour train ride or a $160 flight are the best that I found. Maybe Don has some ideas on the cheap flights forum? Some things to think about….
1. overnight train to Koln, Thalys train from there?
2. Fastest trains are just under 7 hours, not really that bad compared to a getting to/from/ airport, check-in, security total of around 5 hours. Although Munich to BRU airport, if that’s your goal, adds time and complexity.
4. A flight with connections or a train + flight combination will probably take longer than 7 hours.
5. The only Eurlines bus option I see leaves at 16:45 each day and arrives at 6:15 the next — 14 hours overnight on a bus?

[several updates as I thought through this ] …

I did a quick check with Skyscanner, and Ryanair from Memmingen, but not much to show for it. I think with more than 5 months ‘til a late September train departure, you can probably find a special offer on the train that will take a lot of the sting out of a 7+ hour ride.

Best flight I see right now is SAS via CPH at $173 one way and a fairly quick connection.

Ooh—here’s an idea. Use Skyscanner to search all Germany to BRU. Doing that turns up Easyjet from SXF (Berlin-Schoenefeld) showing $47 USD one-way currently. So just take train from Munich to Berlin. You shouldn’t have much problem finding Munich-Berlin trains for a very reasonable price mid-week late Sept.

Actually… no. I didn’t realize the timetable on this route. Daytime durations are showing 6.5 -7.5 hours for Munich Hbf – Berlin Schoenefeld Airport. Yikes. Could you re-arrange your trip a bit to end up in Berlin before going off to Brussels?

Oktoberfest isn’t going to make getting anywhere from Munich any cheaper/easier…. $160 doesn’t really seem that bad.

I know… I think I’ll try to rearrange the dates n’ places a few dozen more times n’ see what I can come up with. Thanks for the help!!

Flying to Amsterdam could be cheaper….. or Düsseldorf / Cologne