Tom,
Here's a bit of an unconventional idea I came up with long ago, not for a bowl but for off axis spindles. It would require some extra material on the bottom of a bowl, more than for a typical tenon..
Instead of turning a tenon on the bottom, consider turning a rough sphere. This can be firmly held in jaws like the Nova 35 or 45mm serrated jaws or in the long nose jaws. Turn the outside first, including the ball. Then, holding the ball in a vertical position, hollow out the bowl. Then loosen the jaws and tilt the piece just a tiny bit and turn another bowl shape inside on a different axis. Tilt again and repeat for more internal bowls, perhaps different sizes, maybe some overlapping.
I've used this method several times on spindles since I thought of it (since it's woodturning, I suspect I'm not the first). Haven't tried it on a bowl, platter, etc but I think it would work.
One of the first was this "crazy golf tee" in this photo, turned from aluminum with a spindle gouge. I first turned a sort of egg-shape on the end of the an aluminum rod, then held that egg tightly in some pin jaws, tilting it in several directions to turn on other axes: Unlike wood, aluminum was basically non-compressible so simply crushing wood fibers to achieve the different axes wasn't practical.
Then in a recent forum challenge, I did the same thing with a wood cylinder.
The pics below show the process with the end sphere on the spindle. This was actually my prototype, proof of principle, turned from y. poplar. I can't imagine why it wouldn't work on a bowl as well as a spindle. (I think I'd first try it with an end-grain bowl.)
Note that only a tiny amount of tilt resulted in a significant axis change.
One nice thing about this method is the holding should be quite secure with the right jaws (for example the Vicmarc shark jaws).
I put some marks on the piece and the sphere so I could have a chance of returning to a previous axis if needed.
Leaving the sphere in place till the end was handy for drilling little holes for some tiny inserts from contrasting wood, just for fun!
Then I parted off the sphere.
I do think this would work for a bowl too. I might try that. A bowl might benefit from a larger sphere, have to think about that.
Note that the holding "sphere" doesn't need be even close to a full sphere, it could easily be just a longish, thick tenon with convex sides and a little extra length for clearance at the top. Rounding the sides would give the jaws a better grip than tilting a normal tenon and would reduce fiber crushing. A larger diameter rounded tenon might be best. Seems to me the amount of tilt needed for an internal bowl in a bowl would be small (depending on the bowl).
Just thinking before trying (I know, dangerous), I'm imagining turning the original bowl with a nearly spherical inside might be a good way to start. And definitely a shallow rather than a deep bowl!
Now I really want to try one! With luck I might be able to get back on the lathe in a few weeks.
JKJ