OK to connect ring terminals directly to a PCB?

Yes this has been done on boards used in "heavy industry" situations where you need a high current rating, without being restricted by the pin spacing (determining the voltage rating) of connectors. There are a few considerations to doing this succesfully

• to avoid pad being wrenched off the board due to the torque forces when doing up the nut, use double sided board with vias all the way round the pads.
• make it a plated through hole, clearance size for the thread you are using.
• wide track on both sides both for the high currents and to stabilise the pad on the board for mechanical reasons, as far as you can go.
• ring terminal shouldn't be put directly on to pad but use a plain washer in between to avoid transferring rotation to the pad, then a split washer or a wave washer before the nut to keep the tension.
• ensure sufficient space between terminals that the ring terminal does not hit the next one in any rotational position

An alternative which works better in most situations, because it only requires access from one side of the board, is quick disconnect tabs, which are available as single, solder in parts. Article discussing relative merits.

One problem is that you must have access to both sides of the PCB when connecting or disconnecting the wire. Also, you need to make sure the screw doesn't extend so far as to contact the chassis below the PCB.

Better to use screw terminals, such as these: http://www.digikey.com/product-search/en/connectors-interconnects/terminals-screw-connectors/1442846