Hi Iain.
If you fancy a challenge, with a little bit of database tweaking, you could create a selection of 'child' items, which you could decide are available at all times, or only of a certain 'parent' piece of furniture is in the cart.
As Eric said, ecart can't do this, it is something that needs to be done with the database, but once the database is altered, ecart can cope very well with the new structure.