'Quite tricky' may be over stating it. If you don't care about variation between one transistor to the next, and you don't care about linearity or temperature sensitivity is acceptable, it can be made to work.
Well it's just a personal experience, but I'll take your reply gladly. In fact, it's something I learnt today. I just constructed a simple range-touch sensing using my own finger connected to the base and a wire. As I slid my finger along the bare wire to the base, the LED grows brighter. It's very tricky to regulate the consistency, that's all.