Bad assumption on the Windows-only part. I made lrc2sylt available for both Linux and Windows, and include the C source inside the distribution. I don't know enough about OS X to know whether it would compile, but my gut says it probably wouldn't.

There is a Perl module MP3::Tag I believe that might help you out, but because SYLT frames are binary data, it's not as simple as entering timestamps and lyrics.

Let me know how you fare in compiling my utility on OS X. Incidentally, I would caution you against using tags that someone else created though, a lot of the LRC files I've found on the net are pretty dodgy...