Comment #4 on issue 2543 by [email protected]: Patch: Allow tweaks in Lyrics. http://code.google.com/p/lilypond/issues/detail?id=2543
One thing that might be worth noting is that parallel music is nothing you would use inside of lyrics, so strictly speaking there might not be a need for tweaks within Lyrics, as \once\override should always do the trick.
However, there is a technical reason for preferring a tweak: \once\override works by recording an old value at the time of override, and restoring it at the end of the timestep. If any _other_ property setting commands intervene in between, this process may lead to unexpected results. Also the property override sequences in the presence of grace notes are not necessarily as expected.
A \tweak, in contrast, is directly tied into the event in question and nothing else. There is no save/restore mechanism involved, no interaction with context properties and no interaction with timing.
For that reason, I consider it advantageous to have it work consistently in Lyrics, even though strictly speaking one should usually be able to do without here.
