Sadly, SQL Doc will only run on a live Database, so unless you redeploy your database from the snapshot using the SQL Compare command line, you won't be able to do this.
Have you considered simply using the SQL Compare commandline to compare the two snapshots? The /snapshot: switch can load the snapshot and the /report: switch generates a report. /reporttype:Simple will produce a simple HTML report. /reporttype:Interactive outputs the interactive HTML report.
sqlcompare.exe /? /v /html > help.htm to generate a commandline html file in which all the various options are described.
You should be able to write a short batch file to generate this report only when the comparison reveals a difference (the %ERRORLEVEL% exit code will be 63 when the databases are identical).
Red Gate Scream (http://labs.red-gate.com/index.php/Red_Gate_Scream
) is a handy, free UI-based tool that allows you to compare two snapshots.
Hope this helps,
Red Gate Software