Ok, so looking at the Pyramid-generated Mako template code, it just literally outputs what's in that config setting, quotes, brackets and all.
So, the syntax has to be:
mako.imports = import logging
with multiple imports as
mako.imports = import logging, some.other.module
not
mako.imports = import logging, import some.other.module
Which I didn't infer from the
documentation that reads "String list of Python statements, typically individual “import” lines" (but maybe I just didn't understand it correctly).