Here is something I found in the Metal Gear 2 ROM image. I never saw this documented so I wonder if it is known already.

Anyway, it works only if you use an emulator so it is not a real cheat, but it looks like the developers left some piece of code used for debug purpose. Here is how to unlock it the hard way.

You can modify the ROM image with an hex editor.
Change one byte at 0x6130 to 0x02 (the original value is 0x00).
If you don't want to modify the ROM image, you can do that at run-time anytime during game (if your emulator allows you to do so) by changing the value at memory address 0xc27a to 0x02.

Doing so you can access some kind of developer debug mode with the following options:

0, 1, ..., 9, A, B, ..., J change game level
<NUMPAD directions> move inside current building
<:> <;> I don't know
<Z> get all weapons and objects
<X> get all cards (last boss?)
<R> get rations
<del>/<ins> I don't konw
<backspace> I don't konw
<^> end sequence
<T> don't know
<P> display position
<NUMPAD .> don't know
<STOP> enter debug mode
<ESC> exit debug mode

Inside the debug mode you can:
<up> <down> scroll the currently displayed page
<0> <1> <2> <3> switch currently displayed vram page
<TAB> enter sprite mode (browse the sprites list with the up and down arrow keys)
<HOME> enter pointer mode (move a pointer on the screen and gets its position with the arrow keys)

And that's all I found so far.
Also maybe there is a soft way to unlock the mode, but I didn't discover it yet.

Anyway, have fun
Joe

Check this Spanish page for all the infos about this debug mode :

http://soft.mundivia.es/mpazos/konami/Metal/Metal.html

There is indeed no way to activate this debug mode by pressing a key for example.

This page is impressive. Thanks for the link.

The guy even found the use of some keys I didn't understand.

Now I look stupid ^^;

These English pages are also very interesting :

http://www.bifi.msxnet.org/?main=konami/gameinfo/rc767.html

http://ips.tni.nl/rom/cheats/konami/

(go down and search for RC767)