This is a great question.
The short answer to your question is that unless someone has gathered the data on all LEEDfor Homesprojects and is willing to provide the average CO2 emission reductions per square foot for all projects, and then compare them to the building codes that were being enforced in that region at the time of construction, no accurate calculator comparing the two can exist.
Perhaps the largest issue at hand is that each LEED-H project is different and as a result each project will have a different footprint.
One of the benefits of LEED-H is that it incorporates the complete built environment. The rating system itself is a bit of a menu of all the environmental facets of building a home. The homeowner/developer/builder chooses which areas of the environment will receive focus from this menu. The result is that one project can focus on water efficiency/reuse while another focuses on energy efficiency and yet another on material reuse and waste reduction. To complicate emissions calculations further ... some projects may focus on all three.
As you can see, regardless of the focus each will have a definitive impact on emissions in a different way, allowing for a wide range of opportunities to affect emissions. However, it does make it difficult to calculate.
Good luck with your research!
For more information:
Read Polly Osborne's Q&A "How much does green construction add to the cost of a new home?"