LO, I don't believe there will be a need to sweep between take offs unless the jet in front overtly trashes the surface during a take off roll and STO. Believe me when I say that this will be very evident to the pilot about to take off but it is rare. There are many advantages to the STOVL design over AV-8 that significantly reduce the chances of any type of re-ingestion - both hot gas and FOD. I'm sure you're well-read on them but the design actually mitigates rather well IMHO.
Even for UK Harrier Ops the VL pads were scrupulously cleaned and regularly health checked because that is a sensible and pragmatic thing to do. A Land Rover with an Air Traffic Control person driving it would inspect the pad immediately after any VL because it was the sensible and pragmatic thing to do in the rare case that damage occurred when operating at home base; it wasn't essential by any means. Talking austere, the UK Harrier Force used to regularly deploy to Norway and operate off frozen runways. One can only imagine the horrendous effects of 820deg C thrust on ice, yet, such deployments rarely had FOD or damage issues because STO and SL did surprisingly little damage to ice, even with that thrust. We would never VL on such a surface unless it was an emergency; my point being that just because we elected not to VL did not mean one couldn't and (in the case above) absolutely did not render the AV-8 non-austere capable as a result. F-35B will be able to operate austere in much the same way so STO and SL will be par for the course if a typical runway length is unavailable with VL being reserved mostly for ship ops and some emergency conditions. VTO is not a requirement.