It would be nice to write a pandoc filter exempli gratia from TeXmacs to HTML.
It would make sense since in this way one does not have to install TeXmacs to convert the file and it would also be a good publicity for TeXmacs.
One needs to know Haskell 
Nobody replied on this topic.
But, your idea is really very good. And I support this idea.
I am new to Texmacs, but with the help of Jeroen, I could use the jupyter plugin prepared by him to run R codes. Now, I can use R in Texmacs documentiation, and teaching to childeren too.
But, sometimes, it is felt to convert to some other formats linke asciidoc, markdown. Pdf, Latex and html formats, Texmacs itself is already creating.
But, link with pandoc is really important.
With regards,
Dr. A.K. Singh