Here's a free plugin for Carrara that might help. It still won't change Carrara's default coordinate system, but it might help convert a bunch of objects modeled in Carrara to a different orientation. 

http://www.sparrowhawke3d.com/TransformsPackagePage.html

I don't know if this will accomplish what you want, but you could send Jeremy, the developer, any suggestions as he has been very open to user input.