Gerd and Andrzej,
I have doubt if the main question here is the tag not be supported by the OSM database, but rather the way mkgmap interprets: internally in the java code or "externally" through rules/syntax in style script.
I also don't think the fact that it is not handled internally by the mkgmap's java code gives the tag a temporary status to be prefixed by "tmp". In my opinion, both rules handled internally in the java code or in the styles can be permanent and definitive.
Therefore, if the tags prefixed by "mkgmap:" are those handled internally in the java code, then I propose simply remove any prefix from this tag, leaving only "fast_road".
Regards,
Alexandre