I don’t know how feasible this would be to actually implement, but would it be possible to define a bbcode ‘block’ which changes the line break rules? (in a plugin)
So you would end up with something like:
This text
is on different lines
[md-linebreaks]
This text
will all be
on the same
line.
This is
another line
[/md-linebreaks]
That way it could be available to power users, without needing any metadata attached to the post 