Allowing sharing a sub-task among parent tasks

Hello all,
I would like to implement a JIRA project for test management purposes, using 2 different issue types, "Test Plan" and "Test Case".

One "Test Plan" can have more than one "Test Case"
One "Test Case" can relate to more than one "Test Plan" (or if you want "Test Case"'s can be shared among "Test Plan"'s
Each "Test Plan" should aggregate time estimates for all the "Test Case"'s associated to it
Things I have already tried:

Create "Test Plan" and "Test Case" issue types; Link "Test Case"'s to the relevant "Test Plan"'s. OUTCOME: Works fine for sharing "Case"'s among "Plan"'s but does not aggregate time estimates
Create "Test Plan" as issue types and "Test Case"'s as sub-task of a plan. OUTCOME: Works fine for calculating overall "Test Plan"'s estimates but does not allow sharing of "Test Case"'s sub tasks among "Test Plans"
Use Structure to organize the hierarchical model. OUTCOME: Works well for calculating overall estimates by aggregating estimates of each child "Test Case" but does not allow sharing of "Test Case"'s among Test Plan (once a "Test Case" is inserted in the structure under a plan, it is not available any more to be added under a second plan)

When I submitted my issue to JIRA support (https://support.atlassian.com/browse/JSP-81631) I was told that this is unattainable at present but that I should request a new feature to make it possible to share a sub-task (in the example above a Test Case) among more than one parent task (in the example above a Test Plan) and still get the parent issue Test Plan estimates to aggregate the estimates of all the sub-tasks associated to it.

Thanks a million in advance,

Marco