Great to hear you've succeeded merging terrasync and osm2city scenery! And I'm looking forward to screenies!
You mention you've tried to get it done automatically, but failed -- could you explain what you've tried?
IIRC, objects are loaded from all paths in $FG_SCENERY up until the level where terrain is found. So I think this should work:
1. create a directory OBJECTS_OVERLAY
2. set $FG_SCENERY=OBJECTS_OVERLAY:VHXX_CUSTOM_SCENERY:TERRASYNC
3. copy (or link, to save disk space) objects -- not the terrain -- from TERRASYNC to OBJECTS_OVERLAY
Since VHXX_CUSTOM_SCENERY comes with terrain, FG should load objects from the the first two paths in $FG_SCENERY, hence 'merge' automatically.
Osm2city is somewhat smart when generating objects: it does not place a new object if there's already one in TERRASYNC, so merging scenery works out nicely.
Regarding VHHH terrain -- I must admit I've never even landed there
if its on the same bucket as VHXX, you're somewhat out of luck unless someone re-generates the terrain. That could perhaps even be me (though I'm not making promises here). If not, you should be able to just remove the faulty XXXXXX.btg from VHXX_CUSTOM, so FG will pick up the correct one from TERRASYNC. You can figure out the bucket in question by navigating the UFO there, placing a random object, and noting the scenery path written to the terminal. Let me know how it goes.
See
http://wiki.flightgear.org/$FG_SCENERYhttp://wiki.flightgear.org/Howto:Place_ ... th_the_UFO