Hi Gerd,
 
 > The apply tells mkgmap to add the tag area=yes to the members,
 > and I think that's what you want.
 
This was my first thought too, but:
 
 > Note that the members of the
 > mp-relation are not the members of the original OSM relation, they
 > are the artifical ways.
 
"Apply" adds tags to original members while I have problems with 
artificial ways. Artificial ways inherit tags form original relation, so 
I would expect, that tags added to relation would appear in artificial 
ways. As you explained, this is no true.
 
"Apply" would work if there were created corresponding artificial 
relation too.
 
I think it's ok, that multipolygon appears as polyline and polygon. That 
way we can create object for boundary and for area. Problem appears, 
when multipolygon has no "area" tag, which is quite possible, since 
multipolygon implies area. I have added a rule to correct this problem:
highway=* & mkgmap:mp_created=true {add area=yes;}
 
See attached patch, which corrects footway, path and steps with area too.
 
Often highway=service is marked as area, but I think it could be usable 
to draw a road around area.
 
-- 
Best regards,
Andrzej