As mentioned at
http://musicnotation.org/mnma/, the predecessor to the Music Notation Project (MNP) was the Music Notation Modernization Association (MNMA). Before Tom Reed founded the MNMA, and before he started publishing Music Notation News, he published Musical Six-Six Newsletter, starting in 1972. Here are scans of the first four issues:
Hopefully in the not-too-distant future, more scans will become available (of this newsletter as well as Music Notation News).
As you can see, Reed was interested in isomorphic keyboards (a term that wasn't in use at that time), specifically ones having two rows of whole-tone scales. He coined the term "6-6" to describe this pattern. He was a violist and piano technician who either acquired or at least had access to a Howe-Way upright piano. (A Howe-Way grand is shown on the cover of the first issue.) That piano's inventor also created a corresponding notation system (
http://musicnotation.org/wiki/notation-systems/howe-way-music-notationby-hilbert-howe/). That might have been Reed's first exposure to alternative notation systems (but I'm just speculating).
I mention all this to give some background in light of Enrique's recent post about the MNMA's bias toward chromatic notation systems. It is definitely true that the founder of the MNMA, like the founders of the subsequent MNP, were interested in isomorphic systems (notations as well as instruments). This may explain the MNMA "screen" requiring notation systems to have pitch-proportionality (though there was no screen requiring the systems to be 6-6).
Doug