I read and thought about it some more. It is far closer to Teletext than I could imagine for the time I have been looking at it.
Most features (40 characters per line, control characters that equal spaces, the identical graphics character set) are on the level of WST Level 1.5 from 1981 (the .5 only added the different local character sets) but with only 21-lines none of which are predefined. The weird resolution is a result of that. The WST Level 2 from 1988 with apparently less commercial success than DCC introduced the redefinable characters that are optional in ITTS and to my knowledge have never been used and are likely not even supported by the ITTS box, so I can will them for now.
I will look for suitable modular FOSS Teletext tools for authoring including art and decoders to modify accordingly.
@drdcc Do you know any uses of the ITTS special characters, that would require an addition to the font. 