It turns out I was over-thinking this, as usual.

The inverter direction is controlled by two relays on the old Boxford driver board, one for each direction.
One end of the coil is connected to +12V. Shorting the other end to Ground activates the relay.

Using the Planet-CNC Output Board, I have connected the NC connection of Relay2 to the Boxford CW direction relay, and the NO connection to the Boxford CCW direction relay.

The common connection on Relay2 is connected to the NO connection on Relay1.

The common connection of Relay1 is connected to Ground.

When the Spindle enable signal is active on Output1, Relay1 connects Relay2 to Ground, activating the spindle.

The Spindle direction signal on Output2 determines whether the NC or NO connection on Relay2 is connected to Ground through Relay1, and hence which way the spindle turns.

No other wiring required :D

Adrian.