I have a remote reservoir on my '33 sedan - fiberglass car, no door in the floor (and I didn't want to add one). I heard the same thing - if your system is in good shape, you won't need to get into it all that often. Probably fine after all is installed, bled, adjusted, not so much fun when you first go to fill and bleed the system. Getting to that master under the floor is a pain. So I added a remote fill reservoir.
My thought is that if you have to get into the master after you have a remote and it's filled, you could take most of the fluid out of the reservoir with a big syringe (or a turkey baster). Also, since most remote reservoirs have a seal at their top, leaving it closed and taking the hose off the fitting on the master, should result in very little spillage. Kind of like holding your finger on the top of a straw that you pull out of your drink, most of the liquid stays in the straw until you lift your finger.