...and pay their class 2 national insurance contributions?
If you register, and pay your NI you're entitled to the full amount of maternity allowance (£137 a week for 9 months) it doesn't matter how much, if anything, you earn. The NI contributions are less than £2 a week. If you pay them for 2 years you'll get the full amount back within a couple of weeks of MA.
So register, sell a couple of bits on test or similar, do a tax return once a year (very simple if you're not earning much) and then claim MA.