... Swiss threads are nothing more than French threading with a left handed thread on the fixed cup [of the bb]...
yup.
you won't want to go the following route for a mirage, but for my grand record, i bought the two separate french threaded campy record headset pieces (on ebay) swapping them for the bsa pieces in a standard headset. saved about $100 that way, 'cause french campy headsets are $200 plus.
also had to mill the fork crown down to fit a standard crown race, but no biggie.
for a mirage, i'd look into the donor bike route. lots of mirages and nomads out there...
i wonder if it's possible to re-tap a french steerer to english thread?