Would it be okay to provide a template when asking reference?
Personally, I would be somewhat offended if a student gave me a template for a letter, and I wouldn't use it.  (Unless I had asked for one, which personally I would not.)

However, it is perfectly appropriate to say, "I believe that the committee will probably be interested in my ability to dangle participles and reticulate splines; it would help if your letter could address this."  It is also fine to include a list or resume of your other activities that he may not be aware of, but leave it up to him how to use this list.

It's quite possible that your professor has written letters for students in this different field before and already knows what is expected.  Also, for letters which are submitted online, the writer normally gets a message from the requesting institution explaining what issues should be addressed.