The funnels are strengthened internally like most liners today and you're quite right, the funnel stays, just like the masts, are purely to achieve that older look.
Regards,
Mike.
One thing you might want to consider is placing auxiliary turbines and generators in one of the funnels. This installation wouldn't be very large - there isn't a lot of room in the funnels to start with - but it might be enough to stick an auxiliary gas turbine and generator. This is what the QM2 has, as by placing the engines up high you don't have to duct the intakes and the uptakes down through the passenger sections. Gas turbines are light enough that you can do this while diesel engines and steam turbines are not.
Storage is also a good suggestion for the volume - as is comm gear.