I am tryng to use a Sansa M240 MP3 player and FM radio on my
motorcycle, with the output going into a helmet sound system
(Starcom1). This means that the headphones -- which usually act as
the FM antenna -- will not be used.

I would like to try using a splitter on the headphone jack, with one
leg to the helmet sound system and the other leg to an antenna. My
first thought was a simple 1/4-wave dipole to run around the fairing
perimeter. Would that work? Would I need to add some passive
components or some matching because the device expects the headphones
to be the antenna?

And, Yes, the Starcom1 also interfaces to a TH-F6A tri-bander.

Ken KC2JDY