I have my IBM 5155 Portable PC booting now, with an EtherLink II nic that has room for an 8KB ROM in it.
Some candidates for what to put in the ROM:
Files for "embedded" PC’s, includes a boot disk maker, a DOS compatible file system, windowing functions and more
Debug Monitor and Tiny Bios for the 8088/8086 Processor (would need to be trimmed down to 8KB)
Ken Yap – What to do with an old XT or AT?