Sounds like MW's "tap under the sink" might work out for you, a pretty good idea and can see that that could be practical in a number of ways. I'll just mention, though, in case it helps in some other way that the "difficult mixing tap" is exactly what I thought Python was potentially good for. They make a whole bunch of adaptors that basically are little brass parts with a thread on one side that fits a given brand and model of tap and has a standard hosepipe thread on the other side.
In my case I had a really odd modern bathroom mixing tap and even when I called Python on the phone they said they didn't have an adapter for it but I became suspicious that perhaps my threads weren't all that strange after all and so I took a chance and bought one of the adapters I though it might be after some study. It turned out to fit perfectly and so with virtually no work I now have this easy system where I unscrew the little screen/aerator from the tap spout, screw on the little brass adapter and then can attach a super-long hose (I bought one of their long clear-tubing ones. It is clean, flexible and not too heavy and attaches to an assortment of gravel cleaner cylinders on the other end.) An unexpeted benefit of my modern faucet was that it has a single lever that stays in the temperature position last selected even when turned off (does that make sense, lol?), so I can repeatedly move my hand from a cup of current tank water to match it against faucet temperature positions. Then when I've matched the right position I just turn it off, connect up the adapter and hose and lift the single lever to slowly increase tap water force, knowing the correct temperature position is still in place. This wastes no water re-matching temps.
Now the whole topic of whether you need any warm water to begin with and whether your hot water heater leaves the water clear of trace metals is a whole extra topic, but in my case the hot water heater is glass lined and I like to do large percentage water changes on a relatively small tank (28g) so this is perfect for my situation compared to huge tanks getting small percentage changes or if I had concern about lots of metal in my hot water.
Anyway, just thought I'd throw this out since you sound like a practical guy and will probably think over all alternatives. You might have a faucet like mine in some bathroom that's farther away but not too far. Doing the whole operation where there is a bathtub can also be handy if you have to "safety-drain" to watch for small fish who might get sucked up and also for setting a bucket of tank water for filter cleaning, and the filter, etc.
~~waterdrop~~
which is about the same distance with a standard mixer on it so that would be a hell of a lot easier wouldn't it. 
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I want one now 
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