The work around you're using should still give you an accurate takeoff (except for maybe the battens allowance), it'll just take a bit longer to draw and adjust if any of the walls are moved around or windows/doors are added, moved or removed.
After seeing your work around there's one other way it could be done which should save you a bit of time. If you use the 'Double Masonry' wall option in the Wall Tool and create a material for the insulation & the plasterboard in the fields where the brick options appear, select 'None' for the other material fields then set the cavity to be 20mm (for the battens) it should give the desired result, however this won't allow for any battens.
Would you be able to let me know if this works for you (if you decide to try it)?
We are planning on looking into updating the Wall Tool sometime this year and giving it more functionality so that you'll have more freedom when creating walls.