In addition to flamethrowers the Byzantines had several other unusual or makeshift weapons of which both written records and archeological finds exist. The problem I have seen is that the AI is not very good in using special weapons, it can be inflexible in its choice of ammunition and uses units like grenade throwers and flamethrowers as skirmishers in open battles. I seem to understand their main use was in sieges, possibly because of supply problems in open battles. I believe, for example, that the portable flamethrower shown in the drawing in post #16 is firing from a siege tower.
Perhaps there could be a submod with special medieval weapons for a bit of a laugh.