can we modify rabbitmq messages using irule

The simple answer is that you can do pretty much anything with an iRule, if you're dedicated enough.

Wikipedia indicates that rabbitMQ implements AMQP, which is a binary based encrypted messaging protocol. (https://en.wikipedia.org/wiki/Advanced_Message_Queuing_Protocol)

The longer answer is that although you can modify any data stream with an iRule, you would need to understand the AMQP protocol to ensure that you didn't break things, for example, if you changed a byte in the stream, or inserted a new or replacement messsage, you may need to also update a checksome or digital signature of some sort in order for the message to remain valid. You might also need to terminate SSL on the BigIP to ensure that you can decrypt the data, and re-encrypt it after making changes.