You can stop a report from processing if you click the X in the data table.  Then you can add segments with out the auto fresh.  Not sure if this was what you're looking for.  It really doesn't pause it, it just prevents it from running until you hit Replace Table.     

You could always build a classfication with ReEx as  round about way of segmenting.   Still would be handy to have in the segment builder.  

Not sure if this would work but you maybe able to hack it in your s-code file there is a list of the top 130 search engines.   Not sure if it would blow anything up  But maybe worth a test.  

We have a hourly report that runs out of report builder.    It runs on about a 1 hour delay (i.e. the reporting coming thru at Noon will not include the 11am hour because the report kicks off exactly at noon and the data has not been processed completely for the 11am hour.   Idea is either.... A.  A...