I guess I've answered my own question; some of my solution may be really sophomoric but I'll mention it here in case anyone else finds it useful. The underlying problems are i) in coming at this from a Flash background ii) the publish dialogue being a bit on the clunky side iii) not being able to find documentation on my specific issue.
So, to get your animation from 7.8Mb to 1.6Mb, bear in mind that HTML 5 does not appear able to use those nice Flash tricks such as having jpeg masks for alpha channels. It seems therefore to be a good idea to sort what background elements you can into flattened jpeg images and accept that any transparent PNGs are going to be pretty much full-fat in your final output.
All of the usual image-specific optimisations still seem to apply, so right-clicking in the library and deciding which images can be highly compressed using photo jpeg is still valid. However, separating jpeg content from transparent PNGs and treating them separately seems to be helpful too.
The kicker though, and I'm guessing a bunch of people have been looking at my previous post, knowing this already, and were just figuring out the polite way to tell me, is that you need to select 'both' when exporting sprite sheets. With hindsight this is pretty dam' obvious, but then with hindsight so is everything. You can probably afford to crunch your jpegs quite considerably, but of course PNGs with alpha are just something you're stuck with if you are animating transparent elements, so the best optimisation available seems to be to contain these components within a single sprite sheet (if possible).
With respect to the clunkiness of the publish interface, the issue for me was simply that within a target folder it will overwrite files but not delete unused content. This turned out to be useful in that my so-called optimisations caused the HTML folder I was examining to jump up in file size by a couple of megs, making the cause fairly obvious.
So that seems to work; HTML 5 isn't quite as bad as I thought it was, and I apologise to those nice people at Adobe for questioning their parentage