While waiting for the load stage rev 2 PCBs to arrive, I'll share some of my thought process in deciding not to use an off-the-shelf isolated gate driver IC on the load stages. Switching the power to the load resistors is arguably the most important part of the entire design, and so a lot of work went in to designing, simulating, and testing it. (The second most important part is probably everything that keeps the inductive spikes from blowing up the MOSFETs.)

Tl;dr: I like making life harder for myself, I'm a terrible cheapskate, and my questionable goal to work down to 3.3V on the DUT's power supply eliminates essentially all off-the-shelf solutions.

Here is the deep rabbit hole I went down again in recently reevaluating the design and deciding not to use an off the shelf isolated gate driver IC with some real reasons, and some retroactive rationalizations:

• It should be cheaper to do a discrete solution (as long as I don’t value my time...), and if I need 50 load stages, costs start to add up. A $1 part adds $50 to the system BOM.
• I thought it would be more challenging, and I seem to like making life harder for myself.
• Most isolated gate driver ICs have an output power supply requirement of 10V+. Since I’m powering the gate drivers from the DUT’s power supply, this would mean the load stages might not work at lower input voltages. 
• There are some with a lower output VCC minimum like 4.5V like the IS480P or FOD8480 that look like they might work but now we’re at $0.93 or $1.67 each respectively in 100 qty.
• Also, 4.5V is still higher than I’d like if I want the system to work down to 3.3V DUT input voltages.
• If I want to turn off the MOSFETs quickly to minimize power dissipation during switching (only for the 0.1R 122A load stage does this matter), I need the driver to be able to sink a decent amount of current.
• IS480P looks like it can sink 160mA+ so that would probably be fine. FOD8480 could sink 80mA which may or may not be enough.
• If that isn’t enough current, then I need to follow the isolated gate driver with a BJT push-pull buffer. However, that then means that my max gate drive voltage is limited by the logic output high level of the gate driver IC. The parts I just mentioned can both swing somewhat close to VCC so that wouldn’t be an issue I think.
• Isolated gate driver ICs seem to have fairly high quiescent current draw.
• IS480P has a supply current of 1.9mA typ. to 3mA max. FOD8480 is similar at 1.6-2.5mA.
• Since the gate drivers are powered by the DUT, and there will be 53 load stages, 3mA each means 159mA drawn from the DUT whether the load is enabled or not.
• If operating with an input voltage of 3.3V is a hard requirement, I don’t see a single isolated gate driver part that can operate that low on the first page of a Digikey search, with Voltage - Output Supply filtered to have a minimum at or below 3.3V.
• This is probably for a sane reason: the threshold voltages of many MOSFETs might be around 2.5V, so there would not be much margin to work with. I never claimed my requirements were completely sane though...
• You'd think an IC would be able to have lower quiescent current draw but since I don't care about the turn-on time at all in my application, I can use high value pull-ups to minimize the quiescent current and come in little lower than an IC.
• The best, more integrated solution would probably be a logic output optocoupler like a TLP2361, followed by a BJT push-pull driver stage. It can operate down to 2.7V VCC, with a supply current max of 1mA. However, its max VCC voltage is 5.5V, which is a problem. Interestingly, the inverting version, TLP2358 can operate 3-20V. So now we need to use the TLP2358 and follow it with an inverter, and then the BJT push-pull stage. This inverter output has to swing to at least 11V if we want to be sure we can drive the MOSFETs gate above 10V to minimize Rds losses, that’s going to be hard to find, and then we’ll also have to add additional regulation to make sure the inductive voltage spikes don’t blow up our inverter. Or we could use a discrete BJT common-emitter inverter but now if we’re using a BJT push-pull driver stage, and a BJT inverter, and an optocoupler...isn’t that exactly where we started?

The result of all of this is that there are no good solutions that meet my questionable requirements. I could add on more and more complexity, or stick with the discrete design that works even if it has a higher quiescent current draw and perhaps isn’t as fast as it could be.