Were we part of a data-gathering program?

IIRC when we bought our 2002's, the first 5 oil changes were at no cost from the Toyota dealer.

At first we just thought this was magnanimous of Toyota. But could this have been part of a program to gather field data from our cars?

Yes this initial service benefit was for 2001-2003 model years in the US at least. It is an interesting question whether Toyota planned to, or actually did collect extra data from these cars. The only two sources that immediately come to mind are engine oil chemistry and HV battery scans. Neither seems very likely to me.

Instead, I think it was represented a certain degree of hand holding for the buyer, and perhaps and effort from above to get the Toyota shops to learn hybrid work.

25,000 miles and no maintenance costs what so ever, what a great way to sell a new technology to a tech-wary consumer.