Thanks guys, all your comments are welcome, even the invitation to go bonkers! (As so often, where a build is for shared use, the design details are shared decisions, and equal spacing is what we have decided.)
I've now done a little experiment and have decided to go with shoulders everywhere - both ends of the bed, top and bottom, front and back. It is indeed easier to cut a scribed shoulder than a perfect mortise. There's no need for machine-friendly methods and it doesn't matter how long it takes (which will be quite a long time, I predict with confidence).
Here we have a wedge shaped bit of scrap ash, about the same thickness as the rails, with a split at the end and a curved line to stand in for the curve on the finished piece. Also a planed down slat which is actually a little bit narrower than the size I want, or what you might also call a "mistake". They'll do for a trial run.
I sawed the tenon shoulders and split the cheeks, then evened them up with the router. The tenon is half an inch thick so there's only an eighth of an inch to remove. This bit of ash is beautifully straight grained and cooperative.
Here's a mortise ready, which as alert readers will spot, seems to have been cut with the wood in the vice.

In my defence, I don't normally do that unless it's (a) on a bit of scrap and (b) the scrap is awkwardly wedge shaped.
and indeed, it fits ok, though the mortise is too shallow at the present.
I realised that at the deepest point of the curve, the mortises need to be rather deep for their width, deeper than I could easily make them by chiselling in the tough ash. I decided to bore them out, like I did earlier. For the real thing I can get some extra accuracy by using the bench drill again, but for this experiment I just used a brace and bit and my nice depth stop again.
and here's a deeper hole
I then cut along the curved line on the bandsaw and tried the newly exposed joint. It fits, but the tenon is longer than it needs to be
I trimmed a bit off. To pursue the experiment to its logical end, I then cleaned up the curve with a nice little boxwood spokeshave
and scribed the shoulder all round
I then pared back to the line
and tried the fit: near enough for something that will be hidden by pillows or bedding
and here's the other side, showing the edges slightly eased
So, it looks achievable. A few fresh thoughts will occupy me for a while:
- This took all morning; with 30 scribed joints plus 30 straight joints, I think I can speed up a bit.
- I can definitely gain some speed by not posting photos of all the identical joint making that will follow after this thinking phase is done!
- The tenons don't need to be very long for the joints to be firm.
- If I'm going to bore out most of the waste on these joints, I reckon I can rely on the holes to get the alignment right and skip the initial mortising in the bit that gets cut off. I can mark the mortise sides after cutting the curve and just chop down straight onto the bench.
- As long as the joints don't rattle, I wonder if I need to glue them at all? Leaving them dry would make the glue-up much simpler - four wet joints instead of 34. I could keep the slats in a straight line by temporarily clamping them between two boards. I can also cut quite generous chamfers on the ends of the tenons.
I still need to decide exactly how the long rails will join into the completed head and foot assemblies. It will probably be concealed bolts and barrel nuts holding stub tenons in place, with something to hide the bolt heads.
To be continued!