Can I use a voltage controlled resistor to adjust high side switch current?
You can use a Voltage controlled resister to give variable current limit .You can also switch in different resister values for different currents .I have used a P channel mosfet as a voltage controlled resister to implement variable peak current limiting on Buck converters using standard hard switching  peak            current mode chips and on descrete component designs that soft switch       .The barefoot P channel mosfet is not very accurate due to on resistance spreads but as temp increases so does on resistance .For most of my jobs this is a good thing for reliability at elevated temperatures like in cars .If you need very accurate current for say a science application then your voltage controlled resister would not be a simple bare foot mosfet .If the negative temp co of output current is not desirable like on say a fan driver then set up at more current than needed and regulate the fan voltage .Summing up if the reason for current limit is really for protection then the simple P channel works very well.If you need accuracy then Spreads in mosfet on resistance will make this concept a production disaster if you just use one barefoot mosfet .Remember that a cheap resister can easily be 1% or better but a mosfet is about 30% .