Hi,
Sure, you could do this with an iRule on LTM. If you're on 10.x, you can use the table command to save this info in a session table. For 9.x, you can use a global array.
You can check the iRule codeshare and table wik page for more info.
Codeshare
http://devcentral.f5.com/wiki/default.aspx/iRules/codeshare
http://devcentral.f5.com/wiki/default.aspx/iRules/high_performance_rate_limiting.html
http://devcentral.f5.com/wiki/default.aspx/iRules/Version_9_Throttle_on_Requests_Per_Second.html
Table command info
http://devcentral.f5.com/wiki/default.aspx/iRules/table
http://devcentral.f5.com/Default.aspx?tabid=63&articleType=ArticleView&articleId=2375
Aaron