I did it!
I thought, I can't be bothered trying to rewrite the entire postbit_legacy template, and besides I'd make a right mess of it, so how about if I just throw some basic tables around what's already there and see what happens?"
And - unbelievably - within 30 minutes and with a few tweaks, it's working perfectly! tested in Chrome, Firefox and IE8 anyway and everything (as far as i can see) still works, including highlighting on inlinemod and the quick edit box.
That's too good to be true, SOMETHING has got to be broken. But I'll wait and see.
Not sure if this is a tribute to the flexibility of vB4's CSS or what.
Of course, it does mean I've got yet another table! I feel like I've got more tables now than vB3 had! But as long as it looks right to the end user, I don't actually care.
The blow screenshot is no longer a mockup, it's an actual screenshot from the test site: