as long as you can find an adequate water pump, I'm all for it.
I had a tough enough time finding mine, and I've already throttled it down. I also got picky about it being external, hehe.
I'm inclined to think that a submersible pump would be simplest, but I would have to do some real research first. Still, I think something could be found. I have a really good LFS and there's always the internet, too.
I did it years ago with Tetra 3 gallon (oval) and a 5 gallon sump (AllGlass). I actually had the 5 gallon behind the main tank instead of below.
Used a u-tube and overflow box on the 3 gallon (took some improvisation) and fed the 5 gallon, then had a powerhead with foam sleeve and polishing pad insert sending the cleaned water back via clear tubing to the 3 gallon.
It made water changes and establishing chemical equilibrium stupid-easy. It didn't accomplish my second objective of a clean, simple look, however, as the overflow was almost as big as the Red Sea (now Palm Azoo) mini h.o.b. I was trying to replace.
I think your plan of a nano sump sounds great, good luck!
Yeah, the overflow box would kind of wreck the clean effect. It would certainly put a downward limit on the size and shape of the nano.
I don't know that I'd trust the siphon if the power went off, too. I've heard that they may be OK, but I'm a suspicious sort.
I'm currently adding the fish population to my first tank and working on putting together tank number 2, so this is not exactly the highest on my list right now, but I would like to tackle the project at some point. I think I'd put a betta in one of these. It might be nice to run three different shapes and sizes of mini nano off the same sump. That's what I'd be tempted to do for an office setup.