I think you misunderstand my point. It would be a simple shifting of existing hull mass into the bulkheads - without engineering it would literally make zero difference as the ship loses mass in one place but gains equal amounts in another. Also, the Anaconda was just an example, the same change would be applied to all ships; the Federal Brickships, for example, would see 44 tonnes of their hull mass being shifted into their bulkheads, while the Viper would see 5 tonnes of hull being reallocated into the bulkheads.
The point of such a change would be to give choice with regards to engineering. Currently, there's basically only a single mod worth considering for the stock bulkheads - heavy duty. Because they are massless at the moment, they derive less benefit from the lightweight engineering mods (a 55% reduction of zero is a reduction of precisely zero mass) , but they instead gain massive benefits on the heavy duty mod as its only drawback they completely ignore (a 30% increase on zero is still an increase of zero). This is not good for choice, as there is a mod that clearly outshines the others. Everyone who is using default bulkheads, whether they are specced as shield tanks, agile fighters, explorers or even hull tanks (although hull tanks may invest in reactive composites), all of them get to enjoy the maximum defensive boost without a drawback. You want to be as light as possible? Engineering mods make no difference to mass with default bulkheads so might as well go heavy duty. Want maximum durability? Obviously, go heavy-duty and enjoy the zero drawback while you are at it.
Shifting some hull mass into the bulkheads would add choice to players, as suddenly the mass variable will be something to consider. Traders, explorers, high-agility fighters and shield tanks could shave a few tonnes of mass by opting for lightweight mods, but it would involve sacrificing the extra defensive boost of heavy-duty bulkheads. Combat ships could opt for heavy-duty mods, but they would suffer from additional mass when doing so. Crucially, those using default bulkheads would no longer have the option for the best of both as both sides would have their drawbacks.