How can I "filter out" a small offset current from a current source?
If you are not too concerned about the output impedance being too low you can add a resistor to ground before the capacitor. For example, if you have 2uA offset and add 470K the offset will settle out at ~1VDC. The resistor decreases the output impedance and the offset voltage reduces the compliance.  

Edit: Like this:-







simulate this circuit &ndash; Schematic created using CircuitLab

You could also add a slow feedback loop to cancel out the average output voltage. 

Or use more accurate parts in your current source to reduce the offset current (perhaps a zero-drift amplifier) and avoid the capacitor.