It seems as though many of the users in these forums are interested in plumbed in machines. Many threads I've seen that are debating different machines contain the phrase "...if only this machine were plumbed in..." when a seemingly incredible machine is not plumbable(sp?). Here's a possible solution:
(NOTE: This has not been tested to my knowledge, and is the product of consuming lots of sugar at 1 AM)

1) Get a plumbing line installed where you want to put your machine. Obviously use a plumber, make sure all materials used are rated for consumable liquids etc etc.

2) Use an assembly like the one found in the tank of toilets that turns on and off the pump (a mechanical switch that is attached to a foam float which rides up and down on a rod. When the water gets high enough in the tank the float pushes the switch to the off position as the water level drops the weight of the float pulls the switch to the on position). This along with an on/off valve to fit onto the of the supply line should do the trick. Its possible that the actual assembly for toilet tanks could be used, though I'm not sure if these assemblies would be ok to use with food stuff, but they can be purchased from a Lowe's or Home Depot for around $20.

3) This assembly should be able to hang into your reservoir attached to maybe a small brace stand on the countertop or whatever the machine is on, and with the end of the supply line hanging into your reservoir would automatically control the water level creating (in effect) a plumbed in machine. Make sure to add a manual on/off valve in back of (closer to the supply) the toilet tank type assembly for cutting the supply regardless of reservoir level.

4) Optionally: Fashion a smaller reservoir out of plastic/PVC to ensure the water is circulated more often, less chance of algae and the like growing, etc...

5) For a more "mad scientist" look: create a siphon system into the reservoir.

PROS:
-Should be relatively easy to rig up.
-Effective, and economical solution to buying a fully plumbing capable machine (I think).
-Not permanent, makes machine available for transport to elsewhere in the house, or other houses for that matter.

CONS:
-Acquisition of food grade materials.
-Might not work.
-Not permanent.

Again, I have not tested it, nor do I know anyone who has. The concept seems pretty solid, and being new to all this, I don't know if I'm the first to think of it/post it. Obviously, all materials must be rated for food/consumable liquids.

I think thats about it. I'm going to get to work on drawings and such for this all tomorrow, and when I figure out how to get the drawings on the computer, I'll post those too, hopefully by tomorrow night/early morning thursday.

-Jeff

Dave documented the "toilet tank" solution to refilling the reservoir in Quit Filling that Water Tank! and I summarized a few other options in Converting a pourover espresso machine to direct connect. Later I plumbed the driptray. I love the convenience.

These two threads discuss doing your float idea:

Converting a pourover espresso machine to direct connect and Quit Filling That Water Tank!

You can also attempt to rig something up with a 2-liter bottle or water cooler bottle, where the neck is suspended into the top of the water reservoir, as the reservoir level drops, this allows air to enter the bottle and fill the reservoir. Not quite as nice looking as the above though. But simple.

Someone also plumbed in a pot filler above the machine's reservoir. not automatic, but convenient. Maybe Dan can find it with his super search skills.

EDIT - Dan beat me to the punch on the above links.