I also have a Sienna and have looked at the ScanGauge. It looks like it provides useful info as part of the standard gauge but, from what I can tell, it looks like to get transmission temp you need to use the XGauge functionality (a way of adding additional codes your vehicle supports).
See below for a link to the current X-Codes: (Sadly, the only Toyota vehicle listed in the extended codes is a Prius)
http://www.scangauge.com/support/pdfs/XGAUGE.pdf