Two things:
- I don't think I've actually encountered a client where style tags must be rendered in the body, but there's no head tag support:
Campaign Monitor seems to agree. In the case of the webmail client you use—which one, in particular, is it? If it's something like Roundcube or Squirrelmail, it may be worth submitting a bug report/patch to the project to add support in the head.
- Depending on how wildly different the emails are, you may be better off making a fully blank template and dumping all body code in via token. If it's somewhat similar, then yes, a template would help.