This thread has run its course. I believe we have agreed on the following:
Enter an application's menu out-of-band with an upwards flick of the Power switch (KeyDown + KeyUp SDLK_HOME) or Start and Select held together (KeyDown SDLK_RETURN + KeyDown SDLK_ESCAPE). Developers of emulators are encouraged to adhere to this proposal as much as it makes sense. For example, arcade machine emulators may have an in-game menu provided by the arcade ROM, so it needs to be accessed with arcade buttons and not an out-of-band command.
Because emulators currently have different buttons or hotkeys to enter menus, some users are getting confused whenever they start an emulator and don't know which buttons can be used to exit the emulator, or enter the menu, and some use the in-set Reset button to exit anything.
I'd like to know how feasible it is to settle on a unified button, or combination thereof, to enter the menus of emulators, and perhaps even of applications that are not just a menu providing an exit function at any time, and get some discussion going about this between developers (who would implement this proposal) and users (who would be the ones using the implementation).
First, we must define which buttons are in use by the emulated systems on the GCW Zero, so we know which are free for use. The GCW Zero has the following controls: D-pad, analog stick, L, R, A, B, X, Y, Start, Select, Power slider raised (as of the 2014-05-05 firmware). Here are the button mappings for emulated systems I know of; please add more as replies to this post:
NES, GB, GBC: D-pad, A/B, Start, Select. This leaves the analog stick, L, R, X/Y (unless you remap B/A to X/B) and Power. Currently, these emulators use L or R for menu entry.
SNES: D-pad, L, R, A/B/X/Y, Start, Select. This leaves the analog stick and Power. Currently, PocketSNES uses Start+Select for menu entry.
GBA: D-pad, L, R, A/B, Start, Select. This leaves the analog stick, X/Y (unless you remap B/A to X/B) and Power. Currently, ReGBA uses Y (by default) for menu entry.
And now we look at the possibilities for buttons for this proposal and their pros and cons.
+: The buttons are right next to each other.
+: It's hard to press both accidentally along with the face buttons.
+: They are microswitches. They are the least likely to wear down after repeated use.
-: Almost all systems use the Start button. Pressing Start first will send Start to the game, causing unintended actions.
-: The buttons are not readily accessible during gameplay. They are separate from the face buttons.
-: They are microswitches. Pressing them will be noisy.
+: The buttons are readily accessible during gameplay.
+: It's hard to press both accidentally along with anything else.
+: Only the SNES uses L and R, but not many games use L and R at the same time.
-: They are trigger buttons. Pressing them will be relatively noisy.
-: They are trigger buttons. They are likely to wear down or stick in one position after repeated use.
Use all face buttons held at once
+: It's hard to accidentally press all four face buttons at once.
+: This combination is accessible during gameplay by pressing the right thumb onto all buttons.
+: They are backed by rubber membranes. Pressing them will be quiet.
-: They are backed by rubber membranes. They are the most likely to wear down or stick in one position after repeated use.
-: If even one button wears down or sticks, this button combination becomes unusable.
Use raising the Power slider
+: It's hard to press accidentally along with anything else.
+: No game system will be using this as a button.
+: It is a slider. Pressing it will be quiet.
-: It is definitely not accessible during gameplay, and players must hold the GCW Zero with two hands just to access it.
-: It is a slider. It is likely to wear down or go out of its track after repeated use.
-: Wearing this button down would hamper any use of the GCW Zero, including things as critical as powering it on and off, setting the volume, and setting the brightness.
-: If held for too long, or held with other buttons, it triggers shutdowns and other actions.