I found a Simpson strong tie product (BC40Z) 4x4 half post base, and it states its intended use is for attaching a 4x4 railing post securely on top of wooden decking. The following picture demonstrates a proper installation.
I got excited about it and bought a whole bunch but then thought after the fact that I probably should have checked to see first if it is acceptable and code compliant to install to install securely on top of composite trex-like decking of about 5/4" nominal thickness.
I can't find any information, even information on the Strong Tie website that states NOT to install on composite decking. It would be so much less of a pain to use these. They are held down with a carriage bolt and 4 16D joist hanger nails, and they are attached to the post itself by 3 16D joist hanger nails on each side.