I think the unlicensed field bit will be your biggest problem.
I had the same issue a few years ago and the student wanted to go up and practise while i was doing another lesson.
When the examiner came he started having a go that it was illegal to send the person solo.
He with an amount of joy phoned the caa to report me. It turns out in the old pre JAR system after a period of time there was a requirement for the person to do solo and sometimes also the X-CQ. I am sure BEagle can give you what the old rules actually were.
Basically the person is flying on your license so as long as they have got a valid medical you can sign them out as long as you comply with all the other requirments ie as if you were sending a PPL student solo.
The only other time i have seen it done is when someone fails there JAR class 2 and then get a NPPL medical. While they are waiting for the NPPL issue you can sign them out. But you have to make sure they don't disappear off to some unlicensed strip and if they do anything daft you will get a right rocket.
MJ