I have a great fascination with "Frankenbike's".

Sheldon has been probably the most useful single source of information, but not all of his contributions can be found on his site.

I recall he had mounted an ahead stem on a long quill stem; someone, with his guidance, had mounted a threadless stem on a threaded stem riser, which would seem to allow the use of two stems; and he had used a threadless fork without a star nut (I believe that is on his site).

A long steerer tube would still seem like the best option, but there seem to be numerous ways to mount two (or more) stems, and various other ways of mounting accessories.