Considerations before the holes...
David Weaver
i remember when Ron Brese made an elaborate shaker bench. It was pretty, and I'm sure there have been folks who have seen the type and thought of the usefulness of the storage under it, but I went a different route precisely due to the situation you're in, but for one additional reason, too.
The first reason was just so I could drill holes all over the thing and put tool boxes under it instead for storage as my at-bench storage is a few planes and small tools. If the tool boxes need to come out so that I can stuff something through a hole, they can be lifted out.
The second reason was that I knew I dreamed of a bench that I could beat. Yours looks pristine on the surface. I don't have the discipline or patience. I don't get stain, oil, glue, chisel marks, etc, on my bench on purpose, but it happens.
I don't often build furniture, but I cut a few rows of half blinds this weekend because the mrs. has a request. They're not as sloppy as they could be, but I cut them as fast as i could because they'll be covered with moulding, and I didn't want to have to pare anything after cutting, so they're ever so slightly oversize. Everyone here knows that they seem to still have a mechanical fit somewhere even with small gaps.
At any rate, I was reminded why I like a wooden screw simple leg vise on the front and why I have holes all over my bench (guitar making doesn't require much, and planemaking requires them only to be near the front board).
I think this is an important consideration from before we even start talking about fixtures - anyone who is thinking of making a bench, some of those posted are fine cabinets more or less with a thick top.
if I had your bench, I don't know how integral the cabinets are to the structure, but I'd think about removing the top one or two to be able to use holdfasts and every manner of gadgetry that fits in common round dog holes.
Just because I would do it doesn't mean I expect you would, too.