Do you mean like:
%%%%%%%%%%%%
\version "2.22.1"
<<\new Voice \with { \remove "Forbid_line_break_engraver"}
% just so this example doesn't run out of the edge of paper
{ \temporary \override Staff.BarLine.transparent = ##t
fis'\breve*50/8 % a quarter is 1/8 of a regular breve, so 50/8
gis'\breve*101/16 % an eight is a sixteenth
\revert Staff.BarLine.transparent a8 b c' d' e' f' g'
a'1~ a' }
\new Staff{ \repeat unfold 7 {\repeat unfold 32 d'8 \break } }>>
%%%%%%%%%%%%
You have to set the lengths of the breve notes manually, though.
> 29 sep. 2021 kl. 20:38 skrev E Appeldoorn <ursus.k...@gmail.com>:
>
> I'm looking for a way of making a note of indetermined length that looks like
> a breve note.
> The note might be 50*4 on the first system, then the system automatically
> breaks. The note then gets tied to the new system where the note might be of
> 101*8 (or any other length). There would be no visible barlines along the
> way..
> An example in real life is the nocturne from Serenade Opus 31 by Benjamin
> Britten.
>
