Sounds sketchy to me. If you put a spacer underneath, it would have to be inserted between the lower headset bearing and the crown race pressed onto the fork. As a result the lower bearing will not be supported properly and you will probably have a lot of play in the steerer. The only way I could imagine it working is if you got a second bearing cap and compression ring (which are normally used on the top bearing between the spacers and bearing) and inserted it on the bottom. Probably still not strictly safe that way either, as you are applying loads to the fork in ways that it wasn't designed for.