An easy way to change who you are at any stage of a game is to enter worldbuilder, save the scenario, which will save to the worldbuilder folder in Saves in Documents\My Games\Beyond the Sword... or whatever. It's just an xml, you look through it and change whatever you want.
Enter Worldbuilder, save the scenario, close Worldbuilder, go ahead and save because you cannot load Worldbuilder saves from the main menu.
Open the Worldbuilder save xml, look for BeginPlayer, Team=0. Change to:
BeginPlayer
Team=0
LeaderType=LEADER_BASIUM
LeaderName=TXT_KEY_LEADER_BASIUM
CivDesc=TXT_KEY_CIV_MERCURIANS_DESC
CivShortDesc=TXT_KEY_CIV_MERCURIANS_SHORT_DESC
CivAdjective=TXT_KEY_CIV_MERCURIANS_ADJECTIVE
FlagDecal=Art/Interface/TeamColor/Mercuriansflag.dds
WhiteFlag=0
CivType=CIVILIZATION_MERCURIANS
Color=PLAYERCOLOR_MERCURIANS
ArtStyle=ARTSTYLE_MERCURIANS
Save, load your game, enter Worldbuilder, choose Basium and you're done! You can even look through the CIV4LeaderHeadInfos.xml file in the Mods\Fall from Heaven 2\Assets\XML\Civilizations folder for any other leaderhead.
Although sometimes it's better to play with or against the guys with the cool leaderheads...