Again, the Riese & Müller Birdy 3 Touring with ETRTO355 wheels is factory equipped with a long cage Shimano Deore derailleur that can handle cogs up to 42t and it works without any problem

So using a cassette with a 39 or 42t biggest cog (BTW these two cassettes, 9-39 and 9-42 give a wider gear inch range than the 407% of the Brompton 3x4 system) is not a problem with ETRTO349 wheels.

The comparison I made is not just comparing the weight of the naked hubs but the weight of the whole transmissions, so the complete 6s Brompton transmission including the derailleur, the levers, the cables, the cogs..., everything I dismounted and replaced by the Rohloff transmission (the weights you mention for the Rohloff are not the weight of the hub but of the complete Rohloff transmission, same as what I listed for the 6s Brompton transmission).

That said, I wouldn't mount a Rohloff on a T-line what would result in the same weight as the current 12s T-line.