Hi Steve,
You could try testing the following:
Put the web scraping check set for alarm and learn but not block in the ASM policy's blocking settings.
Create an iRule which checks in ASM_REQUEST_VIOLATION for ASM::violation_data's Violation list containing VIOLATION_WEB_SCRAPING_DETECTED
Insert an HTTP header in the request or modify the URI to append a query string parameter using HTTP::header or HTTP::uri
Here are a few related wiki pages:
https://devcentral.f5.com/wiki/iRules.ASM_REQUEST_VIOLATION.ashx
https://devcentral.f5.com/wiki/iRules.ASM__violation_data.ashx
http://devcentral.f5.com/wiki/iRules.http__header.ashx
If you get something working it would make a great codeshare example. Else if you get stuck add debug logging to your iRule and reply back with the iRule you're testing, details of the issue you're seeing and debug log from /var/log/ltm.
Aaron