Unfold repeats and 1st/2nd time only notes

[Jan Rosseel]

Hello, 

I'm trying to unfold long repeats (so we dont have to jump back and forth in 
a SCORA score), and that works quite well. 

But there are a few things remaining. 

One of them are 'notes that only have to be played first or second time only, 
typically at the beginning of a long repeat where a phrase that started 
before the repeat ends one or two notes into the repeat, or similarly when a 
phrase in the first alternative ends a few notes into the start of the second 
repeat. 

I'm guessing this just isn't possible at the moment with how repeats and the 
unfold mechanism works. Essentially, I'd need to have some construct that 
takes three (or more!) music constructs as an argument:
* what to display if the repeat is not unfolded
* what to display on the first pass if the repeat is unfolded
* what to display on the second pass if the repeat is unfolded

Anyone have an idea how to hack this into the current LilyPond versions? I 
guess a custom engraver will be part of the solution, but I'm fearing that 
won;t even be enough. 

Thanks in advance, 

JanR