In a product, custom emails may be enabled and edited for a few circumstances.  Is it possible to customize the email about automatic payment referenced on the custom emails page?

As mentioned there, you would simply customize the "Send email when purchase will expire soon?" email as that email will replace the automatic one.

Yes, but that would be sent both to those who need to renew manually and those who have automatic renewal, and would be confusing.  I was hoping to distinguish between them.

There isnt really any way you would be able to do that unfortunately

That was my suspicion.  Thanks.