Create a bat file and run notepad.exe with the command line dialog to open whatever file you want..
For example:
mame_ini.bat
@ECHO OFF
START C:\WINNT\NOTEPAD.EXE /w C:\emulators\MAME\mame_last\mame0110u2b_p3\mamep3.ini
EXIT
(replace the windows directory and mame ini location with the ones relevant to you)
Then go into the Mala options menu... then "Controller>Keyboard>External Applications" and set it as one of the apps, with a key to bring it up. (Remember to change the file type to "All Files" when browsing to be able to select it)
You're going to have to create a seperate bat file for every file you want to edit, and mala lets you choose 3 external apps, so I imagine that should suit you fine