Long notes can be converted automatically to tied notes. This is done by replacing the Note_heads_engraver by the Completion_heads_engraver:
\paper { \translator { \ThreadContext \remove "Note_heads_engraver" \consists "Completion_heads_engraver" } }
which will make long notes tied in the following example:
\time 2/4 c2. c8 d4 e f g a b c8 c2 b4 a g16 f4 e d c8. c2
This engraver splits all running notes at the bar line, and inserts ties. One of its uses is to debug complex scores: if the measures are not entirely filled, then the ties exactly show how much each measure is off.
Not all durations (especially those containing tuplets) can be represented exactly; the engraver will not insert tuplets.
This page is for LilyPond-2.0.0 (stable-branch).