Unsure what your point is ... IIRC both removable BMX V-brake posts can be either M10 or M8 ... it's U-brake posts which are different, they are usually M6 or M8.
Normal removable canti posts should work on an adaptor meant for M10 posts.
Just that a lot of BMX frames already have removable posts, a lot of people ride brakeless. So you wouldn't need to hack anything?
I've no idea about bolt sizes though. I'll go with whatever you say
They're designed for BMX V brake mounts, not cantis