Howland current source with voltage reference
Biasing the load affects the input voltage controlled current since the input voltage is ground referenced, the Bias must also be added to the input with the correct ratio to cancel the effect.

If your CMOS OP AMP input bias currents affect the CC offset, then scale all the R values to lower values and attenuate the input.

 My design here Your mileage may vary.

Design Specs


10uA /Volt CC source  0 to 50uA  or +/-50uA   0 to 5V slider    
Any Bias voltage .  0 to 5V slider.      
Any Load Resistance . Chose 50kohms to 0 ohms (switch)    
Rail to Rail CMOS Op Amps, low noise, low bias current
MF resistors 0.2%


Rev A specs   (assumed, not verified)
Vin offset = 2.5Vdc
Vout offset =2.5Vdc
Single supply OA R2R
Output error with ideal parts 0.2%
Non-inverting current gain   

Simulation results verified here