Voltage or PWM controlled resitor

A current mode PWM controller will effectively drive the LED with a current instead of a voltage. Play some games with the voltage refereance and you should be able to get the range of currents you seek.

I'll see if I can answer your question. First, voltage-controlled current source circuits are often considered, and are a good way to make programmable currents. A simple current source can be created if the load is floating, rather than grounded.

A true or "pure" current source, "sources" current into a grounded load, and this needs a power current mirror as part of its design. ASCII circuit diagrams for both of these have been posted here on s.e.d. multiple times, and are available with a Goggle search.

Finally, let's address your question about a PWM resistor. While I wouldn't use a PWM resistor, per se, for your LED-current source, a PWM programming-signal makes plenty of sense. This can be used to create a PWM current (full on or off with a PWM ratio) into the mirror mentioned above, and if the mirror has a smoothing filter capacitor, then the PWM signal will create a steady programmable current out of the mirror into the LEDs.