I think it would be hard to make it agnostic with all the library code that will be available. All the js packages that are being written for node are written to work with the the feature set and bugs of V8. If there's a V8 node on windows there will be no reason to use a version with any other engine as you wouldn't know what it could be breaking.
Microsoft are effectively ceding server site JavaScript on windows to Google's JavaScript engine without a fight.
There has been talk in the past (from the guy that was in charge and the company that now funds his work on the project) about being more engine agnostic. But it was referred to as a "far future" goal as there were plans for a great many things that were considered more important and useful. So it isn't something that has not been (or will not be) considered.
On MS's past form, I doubt they will cede server-side JS to Google's engine permanently. They have money to throw at people to throw at making their engine and node work together, if they so desire.
Microsoft are effectively ceding server site JavaScript on windows to Google's JavaScript engine without a fight.