Nick, the cruise charts show that vne (indicated) decreases as pressure altitude increases when ambient temperatures are very low. As it warms up, vne doesn't affect max speed in level flight at heavy weights because of approaching transmission limits.
I think the information you're talking about may be embedded in the charts, but it would require quite a bit of effort to draw it out, and I would want to check the legality of posting proprietary information.
If anyone wants to spend $25, AIAA-1990-3008 might have some relevant data. Go to
http://www.aiaa.org.