Another idea might be to support the 4 corners with "posts" similar to those used in decks but of appropriate scale for the sink. Maybe a 2x2 square post in each corner would be inconspicuous enough and structurally sound. There ought to be enough room in the corners to fit some sort of support posts.
I have installers at my house right now working on an undermount sink job. Whatever you do, don't waste your money on undermount sink bracketry. There are tons of modifications hinted to in the installation instructions. Turns out, the cabinetry with pressed sawdust frames in the rear crumble when you try to notch them for the recessed clips. It's a headache... They've resolved themselves to 2x4s but the plumbing is in the way so the pipes are being cut as I type. Wish me luck!