OK, good idea. I won't count my chickens just yet, but one of the team at ICE now knows what I'm looking for.
I've decided that the simplest way to avoid drilling the Hase main boom is to mount seat and steerer bearing with Stauff-type clamps at right-angles. I can also try out different positions, more forward or back, to judge what would feel best.
Whilst there's not much actual progress with the build, the various components needed are turning up - I picked up a new, complete Magura HS33 hydraulic brake set for the front fork last week, for a tenner plus postage. With Julie on the rear rotors, and HS33 on the front rims, the Kett will have the same spec that's on the Lepus I've been riding.
I got some "me" time this afternoon, putting it to good use by sorting through the contents of several boxes. Once done, there was enough space to do something useful - the first thing on my list: spreading the forks from 95mm to 100 OLD.
I've got a nice Deore hub and a black rim to build up, but in the meantime I'll use this no-name silver wheel with a Big Apple on to gauge dimensions.