Is there some sort of an iRule "include" statement?

You can simulate reusable code by separating it into different iRules. You can have multiple iRules of the same event and order them using priority. Then whenever you need to update your iRule, all the virtual's using it will update themselves when you save the changes.

when HTTP_REQUEST priority 300 {
    reusable code
}
when HTTP_REQUEST priority 500 {
    normal code
}