I've successfully installed an EBH on my wife's HiHy and took photos and wrote up an article while I was at it. I think Jason's going to post it here under the "Learn" forum, but until then you can find the link HERE
Was unable to upload the PDF here so you can download it from the HiHychat.com link above.

If you just want to see the photos they're HERE

 

Interesting experiment. We can get some quantitative data. Did you have a chance to record your mileage statistics on your HiHy prior to the EBH? Would be interesting to see how your FE is affected with EBH usage, as well as (if it's possible) any increases to your electric bill...

 

I installed the EBH based more on my data from the Prius. The HiHy is my wife's vehicle and she's not nearly as diligent about record keeping...plus we'd only run 3 tanks of gas through it before I put the EBH in and those were all atypical driving conditions compared to our usual daily commute.

The hit on the electric bill is pretty small as it draws less than 600W and we'll use it on a timer for about 2-4 hours in the morning only in the winter. When we're skiing in Colorado we may leave it on all night...but we don't pay that bill!

 

Just a quick update...I've modified/updated the instructions based upon the experiences and suggestions of two other people who've done the EBH install. Be sure you print out the new instructions before proceeding as there are a few important changes.
http://www.hihychat.com/forums/index.php?showtopic=227