Hi,
A company called SBF in Germany (
www.sbf-triumphparts.de) use to offer a 5.50 x 17" 40-spoke rear wheel for the Thunderbird Sport. 5.50 x 17" is the same size as the Daytona. I don't know if this was fitted with a 180 or 170 (fyi, a 170 was standard on the Sprint & Trophy, while a 180 was standard on the Speed Triple & Daytona, using the same Brembo rim).
I don't know of another bolt on solution, and I think you may need to explore having an oversized rim put onto an existing hub. I would try emailing Gem at CWC here in the UK (see their site:
http://www.central-wheel.co.uk/). CWC built the wheels for Triumph, and offer a rebuild, special build, and spares service. I am sure they can offer a matching rim, new stronger spokes, and either build or supply the parts for a local US build, etc....
None of the above options are cheap!
Ciao,
Geoff