What are the main priorities that are leading you to use a torsion box? I would assume flatness? But what about after that?
Is weight/portability another one? If it's not, you can get away with using thicker skins (e.g. 1/2") and fewer internal supports, OR even get rid of the torsion box design altogether by using other methods to keep a 3/4" work surface flat. After all, the torsion box doesn't lend itself to a replaceable top, but I could see using 1/2" thick top skin with a 1/4" skin that could be attached with screws/bolts into threaded inserts and replaced. The holes in the 1/2" torsion box skin underneath would give you a template to rout out holes in the sacrificial top once it was attached.
As Bert said, if you look at the design of the Paulk bench, which uses 1/2" skins, you find that the internal supports are spaced pretty far apart - not really a "true" torsion box but perfect adequate to keep the bench flat. With some careful planning, the supports shouldn't get in the way of clamping AND worst case you have a line of holes that aren't the best for elbow clamping, but can still be used for other 20mm accessories. More likely, you end up with some holes near the support that you just need to be careful which direction you're inserting the clamp.
