If you were Microsoft, it would be trivial, provided you hadn't based your legal anti-trust defence on saying this was impossible. If you had, for example, done that, then it's obviously 'impossible'. Also, their revenue model only works if they can keep selling you upgrades all the time - no profit in fixing the old shit.
There a lots of pieces of 3rd party software that mess around with IE in ways that MS have claimed are not possible and go part of the way towards what you want:
Once you can run multiple versions of IE on the same machine, and you're controlling which browsers are installed, redirecting traffic to one or the other is just matter of a simple browser plugin on both that has a black/whitelist and redirects to the appropriate browser. Done. Ms could get an intern to code this up, if they actually wanted to.
I had not considered Microsoft's legal need for consistency with statements made in past anti-trust cases. Was the company's argument that IE was not extricable from Windows or that Windows could not function without a tightly-integrated browser? I had thought the argument was the latter, and, if so, I don't see why Microsoft could not provide IE6 as a stand-alone browser while leaving IE8/9/whatever tightly integrated with the OS.
"Also, their revenue model only works if they can keep selling you upgrades all the time"
I think the need for IE6 hurts the velocity of Windows 7 upgrades, especially for large accounts.
It doesn't seem to be hurting them that much, profits wise - and certainly not enough for them to attempt to do anything about it, apparently.
During the trial, I think they argued both of the above, amongst other things. However, I'm fairly sure that's water under the bridge at this point. I would assume it's just not judged cost effective to work on decade old junk, nor to give it credence or attention by doing so.
There a lots of pieces of 3rd party software that mess around with IE in ways that MS have claimed are not possible and go part of the way towards what you want:
Uninstall IE from Windows: http://www.litepc.com/xplite.html
Have multiple stand-alone versions if IE installed: http://tredosoft.com/Multiple_IE, http://www.my-debugbar.com/wiki/IETester/HomePage, http://blog.donavon.com/2009/08/run-ie6-ie7-and-ie8-side-by-...
Virtualise your way out - run different versions of IE in VM's: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=...
Once you can run multiple versions of IE on the same machine, and you're controlling which browsers are installed, redirecting traffic to one or the other is just matter of a simple browser plugin on both that has a black/whitelist and redirects to the appropriate browser. Done. Ms could get an intern to code this up, if they actually wanted to.