NOTATIO

Your idea is to keep staff pairs in the spacing, like 1 tight 2 wide 3 tight 4 wide 5. Is that right? Personally I would adjust that, so that it becomes less pronounced. First and second systems would get a more harmonic look (to my taste) if the whitespace between staves were more evenly distribute...

What about using small open brackets? Like an "L" for the top part and an upside down for the lower?

If this is vocal music with two lyric lines, one above and one below, the lyrics might reduce the ambiguity with the preserved note values.

I prefer that the slur tips do not touch the staff lines (last two slurs in the violin). Maybe this is just an obsession if mine...

The notation of the arpeggiated chord resembles 17th century french non-mesure prelude notation in harpsichord music.

There is also Frescobaldi that has some possibilities: frescobaldi.org
Not really GUI in the way you are after though.

Just to get a feel for how common the use of this marking is, do you know if it is used in other instrumental literature except piano music? These markings occur in other music as well, not only piano music. From time to time I find it really difficult to decide whether to engrave as an regular mod...

But beware, that sign in your example with a curved line at the left does not mean the same as the one OCTO provided.

Thank you both, very interesting. I had a look in Clive Brown's book on classical and romantic performance practice. Apparently the fermata was used in a number of ways, but directly related to this thread would be two quotes from Salieri and Vogler, where three notes in the violin part are under on...

Could you expand on their obvious meaning? As a performer I would certainly solve the situation, but I would not be 100% sure on how to interpret this notation.