To use a macro to modify the processing of the w# markers of a minicalendar, simply add a minicalendar modifier of the form
:w*XX
where XX represents any number from 1-30, in analogy with macro tokens. Note that when you use this modifier, all minicalendar options formatting are ignored, affording you total control over the way each week number is displayed, but also requiring your macro to handle all details, such as making holidays into a second color, etc...
See also: Converting @ with Macros and Converting $$ with Macros.