I've never been asked at immigration, but who knows?

Where were you planning to enter from?

You can always enter from Hong Kong overland and that would solve your problem. Problem is hanging out in Hong Kong is not very cost effective if you are backpacking. Also, if you routing is to go from Northeast to South/Southwest (which makes more sense for a one-way journey) then you probably don't want to enter from Hong Kong.

As for extending your standard 30-day visa while in country, I've only done it twice, but I think there is a maximum number of days (90 days?) you can stay on one visa. 90 days isn't all that much to "see China," so you may have to break your trip into two sections. Central/North to Southeast and Southeast to Southwest/Northwest. In this case, get your second visa when you visit Hong Kong.