I can confirm that the foreign SIM card override works from my experience a couple of years ago.
My T-Mobile had free international roaming baked in at 2G speeds. Unlike the US however, most foreign carriers in developed Asian nations (China/Korea) don't support 2G fallback, so I had free 3G everywhere.
It was pretty much like using the American internet.
Whether you can find one with reasonable data rates in China is probably the main question.
Two that I have used with great success are Kyivstar from Ukraine and China Unicom HK (note it must be HK, not mainland China). Others may be listed at [0].
[0] http://prepaid-data-sim-card.wikia.com/