How many "Relay Outputs" can you dedicate to control the motor speed?
Build a Voltage Dividing Network, Example: put 5 resisters in series (lets say 2k each).
That would only cost a couple of dollars!
Now program the drive to have a min-max speed of let's 30hz - 70hz.
Now wire each PLC relay output in between each resistor, now just program your hmi with a button for 70hz, then one for 60hz then one for 50hz then one for 40hz then one for 30hz, with each turning on the associated relay output.
This would at least allow you to set various motor speeds, changing the resistor network would allow for different speed adjustments.
Just an Idea - this would cost no more than a couple of dollars!!