(1) is possible, but not (2) and (3), (3) being what I want (though (2)
would be nice).

(1)

     * HEADLINE

     Text about main topic.

     *************** INLINE TASK

     Text about tangent from main topic.

     *************** END

     Continuation of main topic.

(2)

     * HEADLINE

     Text about main topic.

     *************** INLINE TASK

     Text about tangent from main topic.

     ****************************** NESTED INLINE TASK

     Text about tangent from tangent from main topic.

     ****************************** END

     Text about tangent from main topic continued.

     *************** END

     Continuation of main topic.

(3)

     * HEADLINE

     Text about main topic.

     *************** INLINE TASK

     Text about tangent from main topic

     ** SECOND-ORDER HEADLINE ABOUT TANGENT

     Text about sub-topic mentioned in tangent

     ** ANOTHER SECOND-ORDER HEADLINE ABOUT TANGENT

     Text about a different sub-topic mentioned in tangent.

    [no return to the first-order tangent]

     *************** END

     Continuation of main topic.