MPS is designed so that allocation can be very fast in the common case as long as the interface to the collector in the compiler and run-time is done well.

They are willing to talk to people about the licensing. They're nice folks, although often pretty busy. CLASP supports using both MPS and Boehm as well.

As for the Open Dylan exception ... yes. But the other thing to understand there is that both MPS and what is now Open Dylan were developed at Harlequin in the 1990s and MPS's original "client" was Open Dylan.