Well done on volunteering to become our first baggage interlining experiment.
I would have thought BM might be able to tell you, if you give them a call.
In the meantime, some unsubstantiated speculation:
- Interlining across two different tickets is a hazardous sport generally
- You can buy a single e-ticket that will cover you BRS-BM-FRA-LH-BKK-TG-SYD - therefore one might extrapolate enormously and dangerously that an interline might be feasible.
- Regardless if you go via FRA (BM1821/TG921/TG920/BM1822) or MUC (BM1845/TG925), you've more than enough connecting time to re-check the bags if you have to.
Let us know how you get on, unless I beat you with an unexpected BRS-BM-FRA-LH-SIN-SQ-SYD in the next month.