In general Multipolygons have rendered correctly in mkgmap since quite a bit of time. Today I discovered one that is rendered incorrectly (maybe this is related to the fact that the tags belonging to it were only written on the relation not on the ways itself so I had to include it via the relations file by adding: "landuse=forest { apply { add landuse=forest; add name='${name}'; add ref='${ref}'    } } "
http://www.openstreetmap.org/?lat=48.2533&lon=16.2278&zoom=13&layers=B000FTF

See the incorrect white square on the screenshot (north-east of Vienna) (actually I tried this with patch v1 and patch v2 of the "fix routing problems caused by way's bbox being too large) and saw no change.