Hmm. I don't agree that whether something is proprietary or 'open' is actually the salient point. I'd argue that actually the relevant point is whether the format is appropriate and necessary. If I can't read a format I don't really care whether it is open or not. I certainly don't think that would change the childishness quotient. (Sending as plain text, however, may well do.)
Does Open Office even have a RISC OS version?
In any case... Lets look at the possibilities and probable outcomes when dealing with someone who sends inappropriately formated files you can't read.
1) Run around and try to read them. Reply in text files.
Probably most people can read word files on other peoples computers, and probably most people would fold like umbrellas and follow this method. However, the bureaucrat won't even know there is a problem.
2) Send a [polite] message asking for the same information in any of the formats you think would be useful. Probable outcome : bureaucrat disregards, or conveniently loses, or forgets to do so, or just gets annoyed and considers the information served.
3) (1), but also (2) regarding future correspondence - bureaucrat disregards your polite message - after all, you managed to read it didn't you, so why put themselves out?
4) (1) but reply in a form you find convenient and suspect they won't (either proprietary or 'open'; it doesn't make any difference to the recipient who can't read it). Now they definitely get annoyed, but presumably also get your point.
5) (3), then (4) on subsequent correspondence.
Now I don't know anything extra about Tarquin's case, even whether he was receiving Word files from them, but if he's done (5) then it may well be the only way to actually get the point across. Whether it is worth doing when being sued I'm not going to judge.