It would have a similar menu to the archetype classes.
On race selection a side menu would pop up and prompt the "Half (race)- Half Dragon
The half Dragon template would be tied to the racial completionist feat. It would include the usual bonuses for a past life (skill, ability bonus, racial enh point), and additionally after each life it would unlock more draconic features. Horns, tails, scales selector, and eventually wings.
The player would have the option to include the features or select a no option so they can decide which features they want to display.
I imagine the wings displaying similar to the aasimar wings but constantly.
Chime in on this one I like this idea that player can choose which race can be half dragon.