I believe this function will do the trick:
phoneui.composeEmail (subject, body, to, cc, bcc, isHTML)
I tested with the code below and it popped up my default email program with the fields filled out (in the simulator, did not test on device).
phoneui.composeEmail ("subject", "email body test", "me@myemail.com");
I also saw there is a phoneui.composeEmailFromPage function in the phoneui.js that may help too, but I didn’t test that one.