Can I parallel multiple resettable fuses to achieve a higher current rating?
I assume you're talking about paralleling polyfuses- this is likely not a viable idea since the breaking capacity will be insufficient to interrupt any reasonable fault current, even if the current were to share nicely. When the last one in parallel opens it has to interrupt the entire fault current. I can almost smell the arcing.. 

You could consider a remotely resettable 500A DC circuit breaker, but I suspect once you get a price quote the customer will promptly reconsider how difficult it is to access a fuse that shouldn't be blowing very often anyway. 

Those are rather decent fuses- low voltage drop, good interrupting capacity (3000A) and fairly widely available.