Take a look at the 'pitlock' security skewers.
I have just fitted two sets, one to our Raven Vitesse tandem (a thing of very great beauty) and the other to my new toy: a Raven Tour, which I have to say I am highly delighted with. Indeed it is also a thing of beauty. And a very practical, stable and efficient machine.
Anyway, to get back on subject. Pitlock skewers.
The standard rear skewer is about 160mm from the underside of the head to the tip of the threaded bit. The 'pit', washers and special anti-tamper nut (much like a car alloy wheel locking nut) can't need much more than 10mm thread. Previous skewers used on the tandem did not have full thread through the nut. The standard Pitlock skewer has plenty.
But for an extra Ģ4.00 or so, you can have a special long skewer which is a total of 180mm long. I know this because I bought one just in case. We ordered them from our 'local' Orbit dealer (who at the moment seem to be sole UK distributors), and decided to cycle over to the shop to collect. A round trip of 98 miles. Whilst we enjoyed the experience, if a bit sweaty on one of the hottest days of the year (last Thursday), I thought it worth the extra at the time of ordering to make sure we had all we wanted. If you add the extra 20mm of the long skewer to the 5mm or so of thread protusion through the special nut when a standard skewer is fitted to a Thorn Raven bicycle, I'd expect you to be able to fit an extra bit some 25mm thick. Would that be enough for you?