I agree with the point that it can't be hoped to be disguised for a lifetime. And if they started with the pretense of having a Bennet son and heir, they couldn't change it back. Male and female lives were so very different back then, that "Lydia" couldn't possibly live her life out as a gentleman and not be caught out. And he couldn't possibly fulfil the role of a husband and father, as already noted.
The only way I could see it It might work would be if the disguise was intended for a few years only... maybe if there was a chance of the Collins line to die out and disappear (and the details of the entail were changed into a way that in the case of all the other branches of the family being extinct, Longbourn might revert back to the female line - however strange it might sound, I understand one could make up an entail just as they wanted to).
I can see the fun idea of a Bennet girl (I understand it has been done with Elizabeth, but I can see how exciting it would be to explre it with Lydia) pretending to be a young man, lving the much less restricted life of a boy/young gentleman. But I wouldn't want any girl to be stuck with no hope for a release and destined to live a lie.