Every so often there is a scandal where a mother publicly announces that her child's father is a celebrity. Last year a woman claimed, apparently falsefully, that Justin Bieber got her pregnant. Is this an entirely modern phenomenon, or are their cases of women doing this in centuries past for wealth and renown?
There has been a multitude of impostors that have claimed nobility. But more in the form of claims to the crown. I haven't read about anyone claiming that their child was fathered by a nobleman but I am 100 % it has happened more than once. Bastards of noble men were common and to my knowledge they didn't get any benefits as they were children from outside the marriage. Nobility has always been very tied to religion so the bastard children were not something you'd easily admit to.
I also think that there might be differences between countries and regions of the world. Say if you had a harem there more than likely be children from different women so what the story is with these children and their "benefits" back then I have no clue about.
I'm no expert at all in this area but here is a list of some more well known impostors.
http://en.wikipedia.org/wiki/List_of_impostors#False_Royal_Heritage_Claims