One thing that is useful to know to work on the lilypond importer is that the code can be run outside of Denemo (much quicker for testing). You make the directory holding the parser the current working directory and put your test lilypond in a file called test.ly there (IIRC) and then launch the parser. It tests for whether it is running under Denemo and if not opens test.ly and parses that.
You will need to be able to read scheme and to have some understanding of lex/yacc lexer/parsers to make progress. What needs doing is, in itself, fairly simple (accepting embedded scheme in the .ly file - that is stuff starting with # would require a neat little call to scheme read procedure, but the rest is just making more rules like ones already there).

When converting from mxml (with Debian Stable's LilyPond) our lilypond parser failed on

\numericTimeSignature\time 4/4

and


\tempo 4=140

both of these could have been ignored, but instead they aborted the whole process.

In addition, empty staffs were generated (a known problem I think that we could just fix after the parse).