The old
EB920 cards had network boot ROM sockets, but I don't think the chips were ever developed.
Now that FlashROMs are fairly common, I imagine one could conceivably develop a mini TCP/IP stack and associated pieces that would mount a NW0: device in time to be selected from Early Startup Control. Would probably require reflashing every time you wanted to change the parameters of NW0:, though.
@ delshay
The 1200 can be booted from PCMCIA with standard ROMs - it's the CC0: device. The only catch is that the actual media has to be one of the 4MB or smaller SRAM cards. Yes, booting CompactFlash or such attached to PCMCIA would require reworked ROMs. If compactflash.device were ROMable, I think that would just about handle it if the CF media was set up as an Amiga RDB disk instead of fat95 via a mountlist.