It is possible to work around Mapnik column type autodetection using a postgres query targeted at it:
select ST_GeomFromEWKT('SRID=900913;LINESTRING(-200 0, 200 0)') as way, ''::text as highway
union all
select ST_GeomFromEWKT('SRID=900913;LINESTRING(-200 0, 200 0)') as way, ''::text as highway
union all
select ST_GeomFromEWKT('SRID=900913;LINESTRING(-200 0, 200 0)') as way, ''::text as highway
union all
select ST_GeomFromEWKT('SRID=900913;LINESTRING(-200 0, 200 0)') as way, ''::text as highway
union all
select ST_GeomFromEWKT('SRID=900913;LINESTRING(-200 0, 200 0)') as way, ''::text as highway
union all
(select your_query order by your_order)