Finally finished!! Since it sounds like you're interested in the same thing, learn from my mistakes.
1) you're right, the TPMS sensors are the same for the steel and the 6-spoke alloys.
2) I bought the "Sienna 2004-2009 16 inch COMPATIBLE Wheel, Rim" off of eBay for $94.00/rim with free shipping. They work fine with the 2010 and look exactly like the OEM rims with the exception that they are a little more shiny than the OEM rims.
3) Although the TPMS sensors are compatible, the lug nuts are not. I found out the hard way when I dropped it off at the mechanic and got a call 3 hrs later that they don't work.
4) You'll need center caps. The OEM ones are pricey so I got some 2.5" Matrix knock-off center caps for cheap off of eBay.
Before:
After:
Close-up:
You'll need Toyota compatible lug-nuts like these I got from NAPA auto:
I got these plastic centercaps from eBay. They were a little loose inside the rim so I ended up putting two layers of masking tape on each of the tabs to make the fit tighter:
Good luck!!