Forgot to put a via near bypass capacitor, and now the boards have been fab'd - what can I do?
Drill a small hole that will allow you to simply run a free thin piece of wire from the ground plane on the opposite side of the board to the capacitor. 

Stripped wire-wrap wire works well. It's 30 AWG, so the hole that you drill can be very small. 

Use a Dremel tool in a drill press if you have such available - I routinely drill #78 holes reliably without breaking the bits. 

Your can modify all of your boards in fairly short order.