In terms of workload, it makes a big difference whether you try to get the existing 6502 BBC BASIC to work on the MEGA65 or whether you implement a BASIC V from scratch. In addition to the open sources for BASIC V for the Archimedes, there are also some compatible versions that are also open source (e.g. Matrix Brandy).
But they all have the common "problem" that they are not available in the 6502 code, but for the ARM2 ... or in C.
In order to implement this on the MEGA65, you probably need a similar or even more time than creating your own BASIC from scratch.
And then the question arises whether a native "BBC compatible" BASIC on the MEGA65 is really worth the effort of building a new BASIC from scratch? I would speak of years of work with, if this can only be done in free time!
There is a BASIC 10 for the MEGA65, which is also continuously improved with the new ROM. Even if it will of course never be a BBC BASIC due to the concept.
So it takes very good reasons for someone to start porting the BASIC V to the MEGA65. This is really a lot more effort compared to porting the BBC BASIC (6502).