There is usually a REASON for a composer indicating a weirdly irregular tuplet. All too frequently, I suspect that reason may be lack of genuine talent. Except for tempo ad libitum cadenzas, show me a nineteen note ligature in Mozart or Beethoven. Uh-huh. So, they lacked inspiration or creativity? I don't think so.
Listeners perceive multi-note groupings as multiples of smaller groupings. A grouping of eight is HEARD as four plus four. Even the four note groupings may be heard as two plus two.
A performer can turn this tidbit of auditory psychology around, and then use it. A performer can blast off a blistering group of twelve by contemplating it as two groups of six, and further thinking of each group of six as two groups of three. So, for the serious performer sincerely attempting to express the composer's dubious intention indicated by a grouping of nineteen, I would say the best method would be to contemplate three groups of six followed (or preceded) by the odd note. In other words, 19 = 6 + 6 + 6 + 1, or else 1 + 6 + 6 + 6.
This comes more naturally to wind players. They commonly articulate rapid passages with some combination of double and/or triple tonguing. So the nineteen note tuplet followed by a half note for a flautist would be
Ta-ta-ka-Ta-ta-ka-Ta-ta-ka-Ta-ta-ka-
Ta-ta-ka-Ta-ta-ka-Ta Taaaaaaaaaaa.
How to break up the note jungle presents a problem for the performer to solve. If it's a scale passage, where are the turning points? If it's an arpeggio, where does the harmony change, or where does the chord begin a repetition? Is there a rhythm pattern elsewhere in the piece suggesting a clue to interpretation?
As a performer, I believe in being respectful to the composer's intent, at least if the composer is respectable. However, the composer who notates a nineteen note tuplet will have a harder time appearing respectable to me. In such cases, I am more likely to "liberalize" my interpretation. It will be a rare audience that will have any complaint.
In fact, if it's a nineteen note glissando, I may even give the passage the back of my hand and let the pieces fall where they may. Is that evil? Sorry, but I don't feel bad about it. ///ADDED NOTE: Nerd bag commentary notwithstanding, I remain unremorseful. I have another name for what others may call “virtuoso repertoire.” I call it *trash*, and don’t play it. Maybe I can't, though that's irrelevant. DSFDF, I suppose.///
Good luck to you in your performance challenges.