@red
I'm sure you can find other spacers to keep the cards apart even with the backplanes off... try it, I belive this could realy benefit from such a construktion.
Heat might not be a problem right away but it's still something to take into considderation, especialy in these hot Amigas that ain't know for their great airflow.
I thought the Prom could take other ethernet cards also. Lowprofile RTL8029AS cards might be hard to come by, I don't know, but I know they exist and it would be great for this.