You would have to be a !!!!!!!! to make up your own GPS approach and descend to 300 AGL on the strength of it, because of the likelihood that:
a. it's not designed to stringent planning criteria and therefore probably not giving you the protection that an approach designed by professionals would do; and
b. seeing as no other bastard would know about it and you're probably keeping it hush-hush because it's illegal, you're likely to plough into someone else as you break cloud at the bottom of it.