Sharing a workaround: Found a way to write polyrhythms with the proper time signature displaying.

Space does not enter rests; it advances through them. Turn on rest entry and force duration and tap either a note name or the letter Y. That will input rests.

Ah! As usual, excellent - thanks so much. Now, where would I have found that in the help files/manual; it was entering Y that did it, and that’s not mentioned there. This forum is brilliant - but I’ve never needed a forum as much; in fact, I can’t think of any other software I use where I’ve needed a forum at all. I know Daniel’s said they are aware that the documentation needs updating - I do hope they consider some of the topics here, since that gives a strong clue about what users need help with.
Despite that small rant-ette of frustration, I am beginning to get used to Dorico; there are certainly a lot of things I like about it.
Anyway, this is getting off-topic now, so I’ll try to disappear for a while.

Well though it’s out of date and thus doesn’t mention Y, it still mentions using note names rather using space.
https://steinberg.help/dorico/v1/en/dorico/topics/write_mode/write_mode_rests_inputting_t.html#write_mode_rests_inputting_t

Dear piano888,
The documentation in the Version History is really accurate and remarkable. I suggest you try there too.

Thanks to both. I’ll follow those up. I did think I’d done - or attempted - a ‘positive’ rest entry, so will have another go later.