Ugh, this definitely does sound like a PITA, I'm not sure how it would be audited in practice frankly.
Where employee amenities are external to the site, the access to the amenities must be sealed, and should be covered to allow for weather conditions.
External paths from amenities to the site are sealed
Here's the thing, there is no requirement to have paths from employee vehicles to the plant sealed, and the risk would be exactly the same in that situation. I interpret this requirement of "amenities" meaning specifically segregated restrooms and locker rooms, that employees are expected to travel between during the course of the workday. If you had a locker/restroom building erected next to yours, employees should be able to travel between them without getting contaminated. Your employees are travelling from "home" to the job site, if you have appropriate controls to decontaminate or whatever on arrival (hand wash, ppe, etc.) then there is no added risk compared to employees coming from vehicles.
Document the heck out of it and keep those bunkhouses (externally) super neat and tidy. Longterm to deal with this requirement I would recommend having at minimum a nice gravel or other dedicated "path" to support your success. I'd also see if you can get it exempted as above. Be clear that it is a separate "residence" and not a locker room.