Modifying external datagroup within an irule

Just to clarify, the command you suggested "tmsh modify ltm data-group uid_optout_list records add { $fool_UID }" is for an internal datagroup, not an external datagroup. An external datagroup get loaded from a file, which is either on the F5 or external to the F5 (but reachable).

There is no way to affect the control plane (datagroup content) from the data plane in v10.x without involving a third party. Your best bet would be to write a log entry, have that syslog'd to an external system like Logstash and have a trigger there which will run a script to ssh to the F5 and execute the required tmsh command.

In v11.5.1 you can streamline this using iControl REST. Also, I would investigate whether iCall/iStats can help you as an alternative.