kind of like forcing a vassalage relationship no?
like many of the clans back in sengoku, to mention one, we have the Mouri family, where Takamoto was a hostage of the Ouchi's to secure Motonari's obedience.
But, what i'm curious about is, is it possible to have the clan refuse to obey just because you took a hostage? we have an example in history, Matsudaira Hirotada refused to "submit" and play along Oda's side when Oda Nobuhide captured Takechiyo but Hirotada refused and insisted on formalizing his alliance with the Imagawa and in a sense, he had forsaken his son in favor of the clan's survival.
So, I think it'd be a nice tweak, having a hostage, but the enemy decides not to give in, and in turn, you have the chance to kill the hostage or keep him as a measure to pressure the enemy, but that won't always guarantee the result.