It is possible to store LavaPossible information within the map file. However, this was never implemented in the mapper's UI.
As far as using (abusing
) the code generator to produce code for setting lava possible, you can make a copy of one of the .tpl files in the Mapper directory. (The .ctl files are information the mapper uses to display units, buildings, etc and to know how to generate code for them, so they can be safely ignored).
Make a copy of default.tpl. The file is split into different sections, which are identified by empty lines with "$$section". (With the exception of $$rem lines which are simply ignored and used for comments within the tpl file).
You could change the wreck section to be used to set lava possible characteristics instead. (Since wrecks are rarely used in a map).
$$wreck
GameMap::SetLavaPossible(LOCATION($x+31, $y-1), true);
If you wanted a special tpl file that would not generate anything else (just used for lava possible), remove / empty the sections such as $$player, $$endplayer, $$unit, $$beacon, $$wall. (I don't remember if you can completely remove the section tags without causing an error, if not just leave the sections empty).
The mapper scans for tpl files in its directory, so any new tpl files you save should appear in the code generation dropdown.
If you wanted to use walls instead of wrecks, just modify the $$wall section. It would be best to use walls, beacons, or wrecks however (units are player specific and this means that lava possible code would not get generated if a certain player doesn't exist).
This is kind of a sick hack, but it works for now.