Yeah... the game only recognizes the head honcho in command - however you may try to drop miss Princess on an emissary of that faction and you'll get the dialogue box saying that she will try to court one of the unwed heirs.

If you wish for her to marry a specific general of your own faction simply take the general out of the his army stack temporarily and drop her on to him then put him back into the army stack.

Regards,
Gilbert de Clare