This is my first project on this site, so am keen to build up my 'reputation', hence I am happy to offer this job at minimal cost to you. I spent a few years designing high power switched-mode power supplies for aircraft, so I know a thing or two about DC power switches.
Regarding your sketch, I don't much like the idea of directly shorting out the PSU with a parallel MOSFET. Even though the switch may be in it's linear region for only a short period of time, you'll need quite a large Safe Operating Area (SOA) for the MOSFET. Even if the PSU folds-back, all of the energy from it's output capacitor will be dumped into the MOSFET. Heat-sinking won't help either, because you can't get the energy out of the part fast enough as heat. I predict that unless you have an enormous MOSFET, or you compromise on your 1us requirement, you will blow the MOSFET off the board. I have in the past embedded MOSFETs into the ceiling by doing what you are proposing!
If you just want to turn the thing off quickly, a series high-side MOSFET would be a safer way to go.
I would be inclined to use a fast high-side N-Channel MOSFET, with a high-side controller chip, such as the LTC7000 from Linear Technology. Remember also that because you have put a diode in your sketch, any residual energy in your 'solar module' probably wont dissipate in under 1us, even if you turn to power off quickly.
If you want me to do the job, I can put a SPICE simulation together to verify my proposal.
Kind Regards,
Ben