I am unsure if the secret character are enabled by default in Gunbird 2 or not, but they definitely aren't in Strikers 1999. From what I've heard, it is not uncommon for the secret characters to be enabled in arcades, if the pic that BBH has is anything to go by.

Here is a list of the codes, taken from a post Haze made on the MAME forum:
--- Gunbird 2 ---
Maintenance Codes:
5-3-5-7-3 All Data Initialised
5-3-7-6-5 Sit an AINE Flag1
Displays "AINE ?" at test, Extra character on Random Box only.
5-1-0-2-4 Sit an AINE Flag2
Displays "AINE OK" at test, Use Aine by pressing "Down" on random box
5-3-1-5-7 AINE Flag Cancelled
Clears Aine from being playable
5-2-0-4-8 Maintenance Mode
Stage Select (Loads of cool stuff here)
Obj Test
BG Test
Play Status
Stage Status
--- Strikers 1945 III / S1999 ---
8-1-6-5-0 Maintenance Mode
0-1-9-9-9 Secret Command Enabled (press up on ? for secret plane)
8-1-6-3-0 Secret Random Enabled (secret plane is in random box)
8-1-6-2-0 Secret All Disabled
8-1-6-1-0 All Data Erased
1-2-3-4-5 Reset Scores