> How could you combine this with Val Shorre’s “Meta II” programmatic parser to make a really extensible language?

At VPRI (Kay's research institute) they did this: COLA. (See also STEPS.)

http://www.vpri.org/

https://en.wikipedia.org/wiki/COLA_(software_architecture)

http://www.vpri.org/pdf/tr2012001_steps.pdf "STEPS Toward the Reinvention ofProgramming, 2012 Final Report"