I built one... well, a prototype for one. I always planned on making it as console like as possible and never quite got around to it. I call it "The EmuCon".
One thing you want to think about, that you didn't mention, was how the interface is going work. Will it require a keyboard? Alot of the work I did on the emucon was eliminating as much keyboard and mouse use as possible. Specifically, every emulator has its own keyboard exit shortcuts, most of which are different from eachother. I used a windows short-cut hotkey combo (alt-f11) to launch a script that kills any running emulators. There are a few other controls I left on the keyboard for navigating the "frontend" (just explorer running in list mode with high-contrast theme, the navigation bar removed, and a top level folder you can't back out of), and the 2600 switches. 9 Keys in total, if you count alt-f11 as one. I plan on eventually building a remote-like device to take the place of the keyboard.
The reason I didn't use something like Mamewah is that I have full sets for all the consoles I support, and some of those names are really long so I doubt they'd show up well.
I see you also decided to go with PS->USB adaptors. You probably did it for different reasons then I did. I did it because I wanted a versitile range of controlers, where I'd only ever have to configure them once for each emulator. Whats more, old PS controllers are cheap and relatively easy to hack (I'll get to that in a minute). You might want to try and find some 6-button layout PS controllers for use with the genesis emulation though.
The 2600 was the most dificult challenge. I wanted to support as much as possible with it, but stella does not allow you to map analog joystick axis for paddles! Its not even like it'd be hard, the code to support analog paddles via joystick is already present for stelladaptor support. So I hacked up the windows regestry to make my PS->USB adaptors appear as "Stelladaptor", fooling stella into thinking they were the real deal. So now I can use hacked PS controllers for paddle games. I even found ways to make Driving controllers and Trackballs work, but they were supported by so few games I never bothered to do the (much more extensive) hacking.
Now my only complaints are the boot up time and the fact that windows boot screens are visible. I'm sure I can fix the latter with a little hacking if I look into it, but you can't really do much about boot up times.
Good luck with your project. If you have questions, I'll offer any help I can.