It's been a while since I did this, but if you program Remote call forward to a port, say for example 28 and then you also set the automatic line selection to grab outside lines, not intercom when it goes off hook, (it probably is already correct), then I think you are good to go.
You need to store the forward number in FEATURE 80's memory spot for ext. 28 in this example.
You then need to invoke call forward FEATURE 11 28 80 where you are forwarding 28 to 80. The trick is as you say, when you unplug the phone, does it stick? I seem to recall doing this but actually invoking it from ext. 10 (admin phone) where I pressed FEATURE 11 28 80 and with no phone plugged in it seemed to work.
Give it a try. Like I said, it's been a while. Also, keep in mind that after a power failure, it will reset and you need to do it again. Being able to do it from ext. 10 is helpful in these cases.
Let me know if you have luck.
Mike