Haven't much time online today, so this will be brief.
I have had one idea that might work. It's very much at your own risk.
There is a way of extracting a PB99IMG.zip from an RUU - one thread in the ATR Guide sticky has this in the title, and the usb unbrick guide refers to it too. Once you have one of those, you can unzip it and it should contain a number of image files (boot, hboot, radio, recovery, system). These are what the ruu installs.
Question is, if you are S-Off (i.e. have a Alpharev or Revolutionary hboot) is there anything stopping you from fastboot flashing these images directly?
You would want Alpharev stock hboot, or maybe the downgrader (don't know what partitions that has - I'd have made it stock myself, but don't know for sure). The stock rom in the ruu won't fit in other hboots.
We use fastboot for updating radio or recovery, so know that's fine (fastboot flash recovery recovery.img for example). We don't use it for rom installation as ROMs aren't distributed as images, but I can't see why it wouldn't work there (fastboot flash system system.img), and boot can be updated that way. And we do this to update hboot. So while the ruu won't let you downgrade hboot, can you just fastboot flash the image? If the hboot itself or the fastboot command doesn't impose the version check it should work.
This is untried territory as far as I'm concerned, and a problem flashing radio or hboot is the end of the phone. So while I think this might work in principle, and as long as the images are intact should be safe, I'm not advocating it. It would be your call.
If you try this, leave hboot to last - when you flash that you will lose the ability to fastboot flash anything else! I would suggest booting the phone before that last step to check it works - as long as you are S-Off we can fix problems with ROM or recovery, but once you flash hboot the only option left if it isn't working is an RUU.
Anyway, not one to rush into, but if the trade in is worth enough to you and you can't run the ruu any other way this might work. It may let you update the ROM anyway!