From what I understand (which, admittedly isn't much) it's not 
possible. There was a thread called "Mac m68k booting process" from 
September of last year that talked about the different problems with 
it.
IIRC there was a person in this list a while ago that was working on 
it but I can't find his posts in my archive (I assume he gave up).

At 9:27 AM +0100 5/27/2001, Matthew Theobalds wrote:
>Whilst we're on the subject of NetBooting and Ethernet cards, is it 
>ever going to be possible to boot into NetBSD direct?
>
>Or would such a move require more information from Apple than they 
>are prepared to give?
>
>All the best.
>
>Matthew

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"Men are born ignorant, not stupid; they are made stupid by education."
-- Bertrand Russell