Heiresses have fathers. Fathers of heiresses have lawyers. Said lawyers say “no-one is signing anything that potentially gives your fortune, Mr Iron-and-Steel, to some random third cousin if the groom proves not to be up to it.”

Possibly I am in the wrong period, but I was fairly certain that you couldn't do that. Bride's money is bride's money and goes to bride's children, and does not come in under the entail.