Ok I have had a long think about this and have come up with two things that might work. The first one is to use an opto isolator. The second one is a bit off the wall and I haven't seen this done before but it seems to work.
It uses a 4 terminal FET as a diode. Current is controlled by a voltage supplied through the body terminal. It is very sensitive so will only take between 1-1.5 volts to get the current you require.
The only problem is the output is negative, but you can easily convert this with an opamp. I haven't looked into what FET to use, you will need to do this. Or the first option might be better.
Thanks
Adam