It is actually pretty easy - although tedious and time-consuming - to make DCG and BAT understand each other.
To start with see the old, but golden DCG tutorial by Scharnhorst:
https://www.sas1946.com/main/index.php?topic=64869.0The 'new' class.dcg (from BAT)
To add a plane in DCG You need to edit the class.dcg by hand using a text editor such as Notepad. Example: this line adds Heinkel He-70F to DCG:
He_70F Heinkel He-70 de 1 5 1 193501 194612 None He-70F None 3000 Meters 300.00 kph Default 300.00 kph
where:
He_70F = Internal aircraft name (from air.ini)
Heinkel He-70 = DCG name (ie. name that shows in briefings etc.)
de = Default country
1 = Default side (1 = blue, 2 = red)
5 = Aircraft type (1 = fighter, 2 = bomber, 3 = transport, 4 = torpedo bomber, 5 = recon, 6 = seaplane, 7= dive bomber )
1 = cannot remember what that is

193501 194612 = available from to (note1: used only if historical prodcution dates is on; note2: DCG cannot handle dates earlier than 193001 and later than 194612)
None = Aircraft upgrade type - upgrades to this type once it becomes available if historical prodcution dates is on
He-70F = skin folder name
None = Aircraft downgrade type, used when a number of aircraft in a squadron drops to zero
3000 Meters = Default transit altitude
300.00 kph = Default transit speed
Default = range
300.00 kph = Top speed
Then you need to add available ordnance in allpayloads.dcg. Example:
Sea_HurricaneMkIId B 4x250lb_Bombs
Where
Sea_HurricaneMkIId = Internal aircraft name (from air.ini)
B = Ordnance type (A = default, B = bombing, T = anti-shipping, D = droptank)
4x250lb_Bombs = Internal weapon name (from weapons.properties)
You need to add separate line for each weapon set you would like to use. Be careful, as the dreaded start-up explosions are usually caused by errors in allpayloads.dcg.
Payloads.dcg is easiest to create using dcg panel by choosing which weapon set is used in different mission types.
Additionally there are the .rds, .rls and .srd files which also provide data to add to the 'locations' and 'territory' files.
Makes no difference if the map is default or add-on. See the tutorial how to make these.
With the size of some of the maps in BAT (e.g. LaChute) is there an easier (i.e. less tedious) way to get DCG to work with a 'new' map?
No.