Two things come to mind. First is that the oil SHOULDN'T cause a problem with just a few simple steps that you already mentioned. The oil will stick to anything it touches so a few well built hay bale barriers should soak up almost all of the oil while the water seeps through - as long as a big storm doesn't cause alot of water to wash right around your barriers and carry the oil with it.

Second is to alert the appropriate gov't agencies that say you are responsible to ensure that nothing gets into their trout/salmon waters. Telling them about this potential problem now might mitigate your legal responsibilities later to some extent and instead put the responsibility on the person doing the blacktopping. They might also be able to tell you if there should be a problem from this big slab to begin with and if they say "no problem" then you would be able to throw some of the blame back on them if a problem does actually arrise.

Good Luck