I built a router gantry system for my assembly table and flattened a big log round for a table project last Summer.  It worked fairly well, although there were "tuning" items to be aware of:  The Router Bit cutting surface needed to be exactly parallel to the glides to avoid lines in the flattened surface.  And Dust in the sliding surfaces would build up if not attended-to, impeding the movement and lifting the cut.  Mine was just waxed wood glides, perhaps a pipe and roller system from online would be better.  I'm not sure if it saves much space however.