Is it possible to permanently hide 1. frame borders in Engrave mode and 2. the frame fullness indicator?
No. All you can do is press the button to hide invisibles (I won’t tell you whick key it is because it depends on your keyboard layout).
Unfortunately the temporary variant is of little use to me. Thanks anyway 
Why do you want to do this?
Unfortunately, I have to draw final barlines manually, which is made more difficult by the right indicator, because I constantly have to show and hide it to see whether I have worked accurately.
I am also familiar with all the workarounds for my actual problem (staff lines without brackets or braces that require a shared last barline), but unfortunately none of them work in this particular case, so sadly I have no choice but to work the way one did back in 1995. 
I actually didn’t want to open this can of worms, but okay The holy grail Dorico is truly not capable of solving my problem. The following issue:
I have a large opera score.
Woodwinds
Brass
Timpani
Percussion
Celesta
Harp(s)
Soloists
Choir
Strings
My goal is not to provide all groups with a final barline, but to do so for each of the listed groups individually. Each of the groups has a bracket or brace and is therefore provided with a final barline at the end of the system. Except for the soloists. They have no bracket or brace. Consequently it is not possible to provide the soloists with such a barline automatically.
There is a workaround with the flat brace, which I can delete in the music symbol editor to simplify situations with more than two soloists on a page. In duet situations, however, this unfortunately does not work, because the spacing that would be required for a flat brace between the staves is too large for two singers - i.e. just as large as it would normally be for harps or celesta, for example.
The coordination line is also no help, as it cannot be repurposed as a barline at the end of the system. Because it reliably jumps to the new page when I try to “connect” the final barlines.
Consequently I have to draw a line manually. And that’s where the fullness indicator becomes annoying.
If you’re talking about drawing a graphic line onto the score, you definitely shouldn’t need to do this. As TonH shows, you can edit the groupings of the final barline easily.
(Though, as a singer, I’d request not drawing barlines between vocal staves…!)
If you’re trying to join the final barlines, but not join the rest of them, then that might be difficult, admittedly.
Only now I mention altering the final barline also joins barlines elsewhere.
Looking in the manual, I think it should be possible for an time signature change, but I have not succedeed yet to make a local time signature change on that final barline
