There is no way to do that today using MARS. The RAW messages are just gzipped files in the archive though, so you could search them yourself. On a Linux box, you could do something like this:

Say I want to find any raw messages with a give username in them during october:

cd /archive-dir/

find ./2008-10*/ES/ -type f -name rm-* | xargs zgrep -i

There are lots of variations on the above. Take a look at the archive directory structure. Each day has its own directory. The gzipped raw messages are stored in the ES directory and start with "rm-". They have the date/hour range in the filename so you can easily narrow down the list of files to search through using date/hour. So, to only look for events in October for the above user that occured between 7-9am(roughly):

find ./2008-10*/ES/ -type f -name rm-*_2008-10-01-0[6-9]* | xargs zgrep -i