To be clear, the legs would support the topper when it’s positioned over the tub, and also support the topper when it’s beside the tub? If that’s the case, do the legs really need to fold away? Would fixed legs be acceptable? Or is there a requirement that the legs be foldable to provide clearance over the edge of the tub?

Does it need to be folding? What if you borrowed the leg idea from something like this?

youtu.be/l2C5NRS1uLE

Hmm. If I’m visualizing this correctly, and depending on the size of the table . . .

Two pairs of legs with stretchers in between, on pivots that allow them to fold up agains the bottom of the table, slighly offset so that the legs end up alongside each other when folded instead of interfering. If you want them to touch the bottom of the tub, set them up to fold at the “knees” rather than the “thigh”, if you see what I mean. The difficult part is figuring out how to secure them in the extended position. If you’re okay with putting in a couple of bolts whenever you unfold, you could add a couple of supports that link the stretchers to the underside of the table at an angle (pivot at the other end again). Or you could attach a length of wood to one stretcher with a pivot and notch the other end so that when the table is unfolded, it drops over the other stretcher and forms a tight cross half-lap joint.

All this requires gluing or screwing hinges or bits of wood pierced for dowels or screws to the bottom of the table to form the pivots.