I think you're on the right lines - the Background Begin/End is your main option here.

One problem with the max-height of the banner image area is that the cropping changes as it gets bigger. Maybe this is OK to live with for now, though using the fixed aspect ratio might be better?

I'm thinking maybe what we really need is to configure a banner in the Theme settings, then have a way to override your default banner on a per-page (or per-application) basis. You could then go wild with banners, and we could make it work in each of the themes in a suitable way, without you having to squish it in using includelets.

I'll think this through and see if I can come up with a nice/clever way of configuring banners which all sites could use, whichever theme they choose.

Joe