That should work for lobsters, yeah.
My naive generalized solution* was to just add sub-commodities together, e.g.
Volturn locally produces 4 units of Luxury Goods and 3 units of Volturnian Lobster
It has a total Luxury Goods supply capacity of 7 units
(even though this completely breaks the illusion of logarithmic increases)
For any market that imports Luxury Goods from Volturn, its stockpiles (in particular, what the player sees as being on the market for sale) will be in the ratio 4 LG : 3 Lobster.
Or Lobsters could have an utility value of 2 in the .csv, in which case it'd be 4 LG : 1.5 Lobster
*Generalized so it can handle commodities a mod might want to add, e.g. there could be a market producing "Carbon Nanotubes" which substitutes for Metals in the production chain.
I don't think anyone would particularly mind if this wasn't supported though.