Hi wildta,
First, saying "the rx400h isn't much different" is true, ONLY as applied to the basics of the HSD. Other than that the SUV version is VERY different from the Prius. The Prius was deliberatly made as light as practical, and it's low drag / shape was deliberatly made to defeat wind resistance. Not so, with the SUV. In comparison, it's a slug ... much heaver, and more drag, and 'over-powered' because it's market is to a buyer that expects more power. I too, wish Lexus' luxury RX sought to economize ... but you and I are in the minority.
Then there's hacking into the HSD's hardware. The 400h's ECU's control how much electric power is managed / used / cooled / charged / discharged. The folks who do this, really have to know what they're doing. Done in a slipshod manner, it'll take you into the realm of irreversable trouble codes ... and at that junction, you can kiss your warranty good bye.
That said, it HAS been done before. Dr. Andy Frank (U.C. Davis Prof) was one of the grand fathers of the plug in hybrid, and one of his first vehicles (that he had his students do) was a mongo Suburban sized land barge. And YES, it was averaging over 100mpg. But the whole 100mpg is kind of slippery. Sure, if you drive X amount of miles EV versus ICE, then you'll get 100mpg. But if you seldom recharge, your mpg may actually be WORSE than a standard vehicle. One of my favorite quotes of Dr. Frank: "This whole 'drill, baby, drill' thing is b*llsh*t"
BTW, using synthetic oil, these early conversions (done over 9 years ago now, in some cases) have suffered NO discernable acidic / sludgy wear ... much less the later ones being done by commercial companies. Some have logged over 100K miles, along the way, doing stints from 20 to 40 pure EV miles. There's some pretty cool mod's going on. But for SUV hybrids? None commercially, when it comes to PHEV's. A few conversions have been done on the smaller Ford SUV, the Escape ... but the mod is not offered for sale. I checked.
http://content.zdnet.com/2346-9595_22-18853.html
Highly appreciate your discussion. Very informative!