Found fairly basic PHEV information at this site:
http://www.autobeyours.com/PHEV.htm

A talented individual working with Prius vehicles has created an individualized PHEV Prius by using a second stock hybrid battery. There are diagrams and photos http://autobeyours.com/rough%20sketch.htm on how he is going to fit the second battery under the floor. The project is a Prius but after reading it, why not the same for an Escape Hybrid?

2005 AWD Escape Hybrid
Best tank trip MPG 39.02 (scangauge II) for 402 miles on I-70, 10.3 gallons used over mostly flat terrain.

Best tank trip MPG 34.6 for E30 for 271 miles along I-80 in Indiana and Ohio.