High side current monitor to measure microamps
Actually the AD629 you already found should work for you. If you look at the web page for it you will see that it is categorized under 'Current Sense Amplifiers'. Its special feature making it so usable is the capability to amplify correctly even with high common mode voltages. This means that both inputs have a voltage with respect to ground, but only a small difference. This is exactly what you need for a current sense amplifier.

When you measure micro amps, the current sense resistor you want to use might be about 1kOhm, which would give you 1mV/1µA (without amplification). You might want to look at Dave Jones uCurrent project as an example for how to measure even nano amps, and for a discussion of how the size of the sense resistor might affect your circuit (though I doubt that even 1V voltage drop will affect your circuit when it is powered by 100V...)

Another option you can consider is using a isolated OpAmp like the AMC1200. You would need to power directly from the rail where you are measuring, so you have no common mode voltage. The isolated output allows you to use it at any other place of your circuit without any level shifting.