Hi Charles,
In fact, we have plans to provide coverage statistics in one of the future versions of TeamCity. You can monitor
http://www.jetbrains.net/jira/browse/TW-934 issue for that.
It is possible to get coverage information and trends using API of TeamCity, but it is somewhat complicated, because coverage support on the server is implemented as TeamCity plugin as well
. You'll have to parse EMMA report files (as we do to show basic info for coverage percentage).