Using a fschange input might be helpful here. By monitoring the file, and setting the option "sendEventMaxSize = N" then when the input checks the file, it will only send the contents of the file upto N bytes. If no content is sent, then you have progress. Not exactly what you are looking for, but without a perl/bash/python/etc script, I don't think it's possible. That is the other option, by the way. Write a scripted input to output the filesize of the file, and perform your search on that input.
http://docs.splunk.com/Documentation/Splunk/5.0/Data/Monitorchangestoyourfilesystem
http://docs.splunk.com/Documentation/Splunk/5.0/admin/inputsconf
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