These are not the best pictures, but I think you will get the idea.
I personally think with some precautions this roof could be done safely. The owner has already told me that the roof is very slick when it is wet.
Unfortunately it is hinged to flip over the edge of the roof making it difficult to use a highlift but I think safety should be your number one concern. You should beg, borrow or rent a highlift platform and not try to do anything standing on the roof! Also suggest that a work platform be installed around the fan for future maintanence work.
In case you can't tell the fan hinges away from the roof out into midair, PURE GENIOUS! I think it is close enough that you can get the top of cleaned while standing on the roof, of course the run off will be at your feet at the same time.
NFPA #96 4-8.2.2
"If exterior fans are located outside the roofline, they shall be provided with a safe access and a work surface for inspections and cleaning."
This says it all, I have some units like this and I refuse to clean the fans, out of respect for my employee safety. Blame the AHJ for signing off on rooflines like this.
FYI, This guy has agreed to have scaffolding set up when we clean this place. This is a first for me, most people would say well just clean what you can or get someone else to do it.