XML base64 namespace and attack signatures

I have a XML/SOAP application where ASM causes a high number of false positives on the attack signatures.

The main culprit is a namespace which is base64 encoded (embedded picture) where the combination of characters often matches a signature.

Is there a way to disable/unblock signatures when they creates a violation on this specific namespace?

I was thinking of fixing up an iRule but I don't know how to make the match between the namespace and a signature violation.