Here ya go.
http://www.vintagevmax.com/vvforum/viewtopic.php?t=1613&start=0&postdays=0&postorder=asc&highlight=
I have looked into hyd brakes for my 92 Vmax-4 and others. Most of what I see others doing is like what you see in the link. They use the stock caliper mount to hold a bracket that that the hyd caliper mounts to. It works great but when you are done you don't have a parking brake. I have figured out a way for my 92 to do the same basic thing as others have done but with a bracket that still allows the stock brake caliper to be functional so as it can be used as the cable actuated parking brake. I just have to find the time to get it done. I'm sure the same could be done with the 540.
As said in the link, most of the newer Yamaha hyd master cylinders fit the older sleds almost perfectly and the electrics plug right in so that is the easy part. If you need a longer hyd hose get one off a 4 stroke Phazer. They had taller bars and the hose is long enough to rotate the caliper around the back side of the chaincase on standard bar hight sleds.
opsled
PS. I don't know about now (summer) but I bought a complete setup (master cyl, caliper, rotor, hose, P-brake cable and caliper) from a late model trailing arm sled off of ebay for under $50 shipped. Figure out a good bracket setup, maybe a longer hose and you're in business.