PeterW, regarding your question concerning my comment "the only problem I see is that most control circuits are AC and diodes don't work the same way in AC", I can assure you that diodes work the same way where I live and elsewhere too, even in Canada and the UK!
Now I'm joking, if you please!
What I meant to say is that, in general, control circuits are 120VAC or more recently, 24VAC. I didn't mean drive control circuits. Drive digital inputs have been 12 or 24VDC as long as I've been associated with drives (18 years). For 120VAC control, you needed to add interposing relays.
Regarding plchacker's relay question, it seems to me that the situation is being made more difficult by leaving the drive in 3-wire start/stop mode. Why not switch the drive to 2-wire maintained start/stop and use a relay with the two panel pushbuttons to covert 3-wire pushbutton to 2-wire maintained. Then take the second pole of the relay and connect it to the start/stop drive terminal. Also connect the PLC start output to the drive start/stop terminal. And connect the PLC reverse output and the reverse switch both to the drive reverse terminal.
So, you have two wires on the start/stop terminal, one from the PLC and one from the latching relay. And, you have two wires on the reverse terminal, one from the PLC and one from the reverse switch.
As I see it, that would be one relay. You may need a switch for PLC/Manual selection, as well since you need to power the latching relay when the PLC isn't there.