Following draft-langtags (and CLDR usage), it would be "tl-Tglg-PH"
Of course the Tagalog script, unless it is enjoying a renaissance, hasn't been used much in centuries.
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Michael Everson * * Everson Typography * * http://www.evertype.com
Following draft-langtags (and CLDR usage), it would be "tl-Tglg-PH"