Here's a layout for the Tagalog (Baybayin/Alibata) keyboard. Extract and move the two files to ~/Library/Keyboard Layouts/, then go to System Preferences/Language & Text/Input Sources and check “Tagalog”.
I tried to follow Paul Morrow's font convention as closely as possible, but I kept the punctuation marks and digits and moved the Spanish kudlit/virama consonants under Alt/Option (you can still hit Alt-= to get the virama separately).
Update, 23 Feb: Newer version, with Ra as U+170D (per Paul Morrow fonts) and Shift-Ra as U+172D (per current Baybayin Modern Fonts). Alt-, maps to the single vertical bar, and Alt-. to the double bar. Default X maps to “k+s”.
Update, 14 Oct: Version 1.21, with missing function keys.