The number in the triangular boxes indicated the number of beats the conductor gives in the bar (4 beats in bar 80, 3 in bar 81), the beats are irregular and indicated by dashed lines.
This page, at least, looks pretty straightforward to me. Without the wider context of the rest of the piece it’s impossible to say whether or not this approach would hold true for everything, but I would be tempted either to work in open meter and add dashed barlines where needed, or to add aggregate time signatures (e.g. the passage between 80 and 81 could be represented as a 4/4|4/4|4/4|2/4 time signature, which would create the dashed subdivisions and also count as only one bar). The only notational aspects that would trouble me would be the cross-staff beaming in the partial bar 79 we can see: those instruments would all have to be held by the same player and all be melodic in order to share a beam; and there’s no built-in support for the kinds of thick black lines that connect the notes in the double bass staff, though users have taken to using hairpins with a more substantial line thickness and no aperture to approximate that until such time as Dorico can do it “natively”.
Dorico doesn’t support triangular enclosures for text, but it should be reasonably simple to superimpose a suitable triangle and a number in two Shift+X text items with the ‘Avoid collisions’ property deactivated.
Shift-X items are not System-wide, correct, only per player? This would need to be a system item as it’s a time signature… hence I was thinking of using it as a tempo marking… can you use custom fonts for each tempo marking?