Is it possible to make a platform that hangs out over the front wheel, but is attached to the frame rather than the fork, so it doesn't affect handling? Would it be a good thing to do? Anyone done it? How?
Steve, fascinated by options...
[ack, must have typoed when editing the subject, meant "mounting stuff from the frame over the front wheel..."]
wouldn't be too difficult to rig something up if you can weld. It will still affect handling because of weight distribution, but I used to ride a royal mail bike with a massive platform out front (attached to the frame) It was no problem to have girls sit up there...
Bruce Gordon Ti Rock N Road , Fat Chance Mountain Tandem 
My elderly F-frame Moulton Stowaway has a front rack fixed to the frame not the fork. Initially it was disconcerting to have the wheel move separately from the rack and bag hovering above it. The separation of fork and rack makes for crisper unweighted steering. There is no weight movement from side to side.
Hembrow make one that cantelevers off the downtube for extra strength.
Most of the frame-mounted luggage is on smaller -wheeled bikes. The Pashley postal bikes are 26" but are specially designed from the start to accept the rack. The head-tube is quite long and the metal quite thick.
The Brompton 16" wheel folder has a special tab brazed to the head-tube on which you can mount various luggage options