Here's another example of an 8MM6 map, 216 x 216: http://alexspl.narod.ru/216_Underground.h3m
I've figured out how to enable support for 216 x 216 maps without replacing the existing sizes, and also how to address most of the issues listed in point 3. I'm patching it manually, so I won't be able to work on other versions until the beta for Book 4.0 is ready.