Josh, a similar question was posted on the Google MapInfo-L list awhile back. Have a look here at the thread (hopefully you'll be able to see the thread?). As you'll see, my suggestion of using the convex-hull option didn't work(!), but he went with a solution based on creating voronoi polygons based on the centroid of each line and pline.
That's pretty much what I ended up doing in FME:
- Shortened all streets by 5% on either end
- Inserted extra points at 100m intervals
- Replaced the lines with their individual points
- Created Voronoi polygons for all the points
- Dissolved those Voronoi polygons based upon the FIRE_L attribute
- Clipped the whole thing to the accumulated bounding box of the original data (because the Voronoi polygons along the edges stretch quite a distance away)
There was some weird stuff happening around the (elevated) freeways, as sometimes overlapping ramps had different FIRE_L attributes, but apart from that it worked reasonably well.