Finally, I've tracked down an acceptable underwater housing for my Minolta G400. It's a DiCAPac WP-400 (from Digital Rev's eBay shop @ £21.99 shipped to the UK).
The WP-400 is basically a sealed heavy-duty plastic bag with a port for the lens. It's sealed with a zip-loc closure, which is then rolled over several times and held shut with velcro. The manufacturer lists a number of cameras it's claimed to be compatible with, but neither the WP-400 (or any other of their other sizes) lists the G400, so I took a bit of a guess. The camera fits in it OK, but the case design has the lens port slightly off centre, whereas the camera has a more-or-less centred lens. At wide angles the port can cause vignetting if the camera isn't in exactly the right place, but it's manageable. The main advantage of the EWA-Marine case is that the lens is enclosed in the port, so that when using the flash, there's less risk of it reflecting off the inside of the case and causing unwanted artefacts in the photo. (The EWA-Marine case basically has a sheet of acryllic across the entire front of the case - so there's no isolation between the flash and lens.)
My next project is to work out how to trigger an external flash (I don't think I can get an optical cell and Skyport in the DiCAPac case as well) and build a case for one of my Nikon Speedlights.