One way to incorporate Jews would be in a manner similar to the patricians in republics with the exception being that they are tied to castles and cities, not so much bishoprics. Doing to would allow them to remain landless of sorts, but playable just as republics are playable.
They would serve as the bankers, merchants, scholars or the realm, with the potential to become extremely wealthy, but not too militarily powerful. They would have limited ability to conquer lands, perhaps CB or events for a return to Israel, but such would require a unity of many Jewish communities and leaders to achieve.
The character would be able to play as a religious figure, seeking high office within among the religious class; a merchant or possibly a scholar or teacher or a general to lead troops. Each of these "careers" could be used to advance through the the ranks of the Christian/Muslim/Pagan ruler's who control the county. For example, a scholar can be used to tutor the children increasing their learning and whatever other skill they excel in. They would serve separate from a guardian, more like a tutor in a specific field. For the non-Jewish player, this would allow a new resource of learning, economy and technology to tap within the realm. Should they rise to such a highly respected position within the realm, one can grant them lands and titles. However, one can also expel them from their lands, seize their assets and essentially gain a short term boost to gold at the cost of long term technological and economic growth. One can also invite Jews to their realm at the short term increase of revolt risk, but with the long term benefits.
Playing as a Jewish character would be less about acquiring landed titles and more about gaining prestige and wealth. I am not entirely sure how to implement this. An example would be choosing whether to provide your non-Jewish liege with troops or money for his war.
Obviously this is not comprehensive, but combined with other ideas would make adding Jews worthwhile.