Completion_heads_engraver for line-ends only?

[Graham King]

I'm preparing an edition of de Wert's motet "Ascendente Jesu in naviculam" 
which has an extended stretto section with dotted rhythms across barlines.  For 
this, Harm's Mensurstriche example in the Learning Manual produces a mostly 
beautiful result.

However, for the sake of those singers who, faced with this passage, find 
themselves <ahem> all at sea, I would like to avoid notes extending across 
line-breaks.  The idea is to use something like the Completion_heads_engraver 
that would take effect only at those barlines that coincide with a line break.  
Does such a thing exist?

As a secondary question: Is there a straightforward way to avoid collisions of 
mensurstriche with beams?