In the interest of invoking Cunningham's law, I'll nominate "natural" danish oil for depth with a couple coats of polyurethane on top for durability.
It’s not the most durable, but when I think hardwood and antique I think shellac.
Hard wax oil could be an option, which should make the surface more wear resistant.
Are you planning to just put the plywood on top, or will the original desk top still be visible so you want to match the finishes?
Just putting it on top. Should there be anything put between the original desk and this top?
The plan is to use some cutoffs to create a 'frame' so it cannot move (3 hidden sides) and also to then create essentially a clamp under there as well so it is reasonably stable.
This is not meant to be furniture quality, just nice and large enough for me to enjoy in my basement office and to support multiple computer monitors.
I just put 3 coats of polyurethane on my workbench plywood and it's doing fine after a year or so. I'll probably switch to water based poly when my can of oil based runs out since I hear it's just as durable indoors and easier to clean up after.
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