(Terminology [from Lilypond documentation]: Mensurstriche (‘mensuration lines’) is the accepted term for bar lines that are drawn between the staves of a system but not through the staves themselves. It is a common way to preserve the rhythmic appearance of the original, i.e. not having to break syncopated notes at bar lines, while still providing the orientation aids that bar lines give.)
Mensurstrich support is made of two parts:
1) More flexible bar lines: between staves, across only part of staves, no bar lines at all (see also this other feature request ).
2) Cumulating tied notes across bar lines into single (possibly dotted) notes, whenever possible.
Part 2) could be dynamical and mostly a matter of displaying: notes and rests could be stored internally as they are now (i.e. separated by measure boundaries and tied: no need to change the current concept of measure); they would be 'simply' drawn as single notes if Mensurstrich is turned on.
Not all combination of tied values can be cumulated (brevis + semibrevis can (=> a dotted brevis), brevis + minima cannot); when they cannot, they would be displayed as now.
Lilypond supports Mensurstrich (Lilypond doc page ).
According to its documentation, Sibelius supports 1) natively, but requires tricks and manual adjustments for 2).