My M0131 does not boot my 64k ROM 512k either.

I haven't made one of those CapNet things, but I'd be interested in hearing about any experiments with building one. The DE9 versions of the phone net adapters are slightly more difficult to find, and more expensive than the minidin8 ones. Just of the top of my head, adapting the minidin8 to DE9 should be entirely passive, since it should be the same signals.