I ended up setting “layout options - bar numbers - minimum distance from staff” to 3 3/8 to prevent the bar numbers from colliding with the ottava treble clef, but now the distance from the normal treble clef is too big…
another option might be to make the “minimum distance from other objects” option respect clefs… related thread: https://forums.steinberg.net/t/bar-numbers-distance-to-different-clefs/118926/8
Thanks for the suggestion, which I’ve made a note of as something for us to think about in future.
It seems like it’s really just a missing setting with this collection of Engraving Options:
Adding a Minimum gap above treble G clef with 8 above setting would be a useful solution.
Thank you @dspreadbury and @FredGUnn !
While we’re at it, maybe also consider a setting for percussion clef? 
An example:
Notice also how the bar-region bar numbers below the multibar rests in the 1-line staves are distanced unevenly…
I have the minimal bar line protrusion set to 2 spaces for 1-line staves; this pushes the bar numbers too far down for my taste – however, in the first staff, the distance appears much nicer… could this be a bug where the distance is calculated differently if there is a change from 5-line staff to 1-line staff in the same line? This “bugged” calculation actually gves a much more consistent result in my opinion!
This setting was introduced in the recent version 6.0.2, which makes me very happy!
For reference: the smaller gap above the percussion clef I mentioned in my post above can be achieved by simply adjusting the minimum distance for bar numbers in Layout Options.
…though the inconsistent resulting distances for bar number ranges between 5-line and 1-line staves shown in the image above are still problematic. Even more so when using old-style multibar rests as shown here:
