iRule to read dynamic file in /tmp/ location

Tried to update my original answer, but it wouldn't save, so here's the idea.

You can use the following commands to work with data groups from the command line:

 CREATE
tmsh create / ltm data-group internal TEST_DATA_GROUP records add { MyKey { data MyValue } } type string

 MODIFY
tmsh modify / ltm data-group internal TEST_DATA_GROUP records replace-all-with { MyKey2 { data MyValue2 } }

 LIST
tmsh list / ltm data-group internal TEST_DATA_GROUP

In your iRule, you could grab the value like this

set myval [class match -value "MyKey" equals TEST_DATA_GROUP]