Used regions in PSO GC:
80004000-80004194 (0194) ExtendedItemInfo (AR code uses 80005000 due to conflict with q8853x)
800041A0-800041B8 (0018) UnsellableRares
800041C0-???????? (TODO) EnemyDamageSync

8000B06C-8000B088 (001C) BugFixes
8000B088-8000B0E0 (0058) BugFixes
8000B0E0-8000B254 (0174) q8853x send_function_call handler
8000B5C8-8000B5DC (0014) BugFixes
8000B650-8000B6BC (006C) EnemyHPBars
8000B86C-8000B8C0 (0054) FlickeringStatusIcons
8000B938-8000B958 (0020) ItemPickup
8000B958-8000B9D8 (0080) Palette
8000BAB4-8000BB50 (009C) CommonBank
8000BBD0-8000BBF0 (0020) BugFixes
8000BF30-8000BF5C (002C) HungryMagSound
8000C3F8-8000C474 (007C) BugFixes
8000C640-8000C654 (0014) BugFixes
8000C660-8000C688 (0028) RareDropNotifications
8000C690-8000C6BC (002C) RareDropNotifications
8000C6D0-8000C6F0 (0020) BugFixes
8000C8A0-8000C8B4 (0014) BugFixes
8000C8C0-8000C8D0 (0010) BugFixes
8000CA40-8000CA80 (0040) Palette
8000CD00-8000CDF0 (00F0) Palette
8000D6A0-8000D6BC (001C) ChatFeatures
8000D980-8000D994 (0014) BugFixes
8000D9A0-8000D9B8 (0018) BugFixes
8000DFA0-8000DFE0 (0040) DrawDistance
8000E1E0-8000E1FC (001C) BugFixes

Used regions in PSO XB (40EU):
002DB550-002DB600 (B0) handle_0E => EnemyHPBars, FlickeringStatusIcons
002DC970-002DCA70 (100) handle_91 => EnemyDamageSync

Potentially available regions in PSO XB:
002BFDB0-002BFE40 (90) handle_03_phase2 (replace handler in table with do_nothing, or just prepend with ret)