I have played as de Bouillon kings of Jerusalem without cheating. I started as County of Boulogne, then inherited Duchy of Lower Lorraine and conquered the Holy Land during a crusade. It has certain difficulties, but nothing which can't be sorted out if player is careful enough.
Count of Boulogne is Godefroy's father at the start of 1066 scenario. Duke of Lower Lorraine is childless and his primary heir is his aging father, who is unlikely to sire more children. He isn't a problem as he is more likely to die before his son does. Next in line of succession is Godefroy's older brother Eustace, who is also quite capable character. If Duke of Lower Lorraine manages to sire sons you must assasinate them to secure your succession. Otherwise you can just focus to staying alive and developing your only province. Your French liege can give you some protection against the potential Norman threat. If you don't want to play as Eustace and want Godefroy to become the primary heir of Lower Lorraine then you have to also assasinate Eustace and risk gaining kinslayer trait to your count. Once Eustace or Godefroy inherits Lower Lorraine you can switch to him or keep playing as the old man till he drops dead and Boulogne and Lower Lorraine unite by inheritance. At this point you will become German vassal instead of French vassal.
Then it's the time to take second step of the plan. Personally I would wait till the Crusade is called againt the Egyptians. It doesn't matter if it's against Alexandria (better as most crusaders and Egyptian forces are in the Egypt and Holy Land is rather unprotected) or Jerusalem (you need to act fast if you want to take the best provinces to yourself). German liege is likely to join the crusade once you declare war against the Muslims, so it might be wise to move your troops nearer to Holy Land before you declare war, unless you want to arrive to the Holy Land and find out that your liege is already there besieging the best provinces. Take the small sheikdoms one by one and avoid the main Muslim armies. Hopefully you were fast enough to take enough provinces so that you can create Kingdom of Jerusalem. Once your goal is achieved make peace with the Muslims and start saving money for your title. During the first years, before you have opportunities to expand you are quite vulnerable as your provinces are looted and wrong religion and bringing more troops from Europe takes a long time and is very expensive. Key to success is careful expansion by conquering small independent Muslim sheikdoms with the troops you can recruit from the Holy Land. Once you're strong enough you might conquer other crusader realms (although if they still have European lieges it might be quite difficult) and defeat the great Seljuks and Byzantines.
