IUS 11.1 and extern/import tasks inside an SV-interface?

I'm trying to figure out a way to give my packaged classes indirect access to tasks that are declared as hierarchical-pahs.

Basically, I need a standard class definition that can access multiple instances of some module-instance within my testbench. Example:

module legacy_module_1;

task xxx;

endtask

endmodule : legacy_module_1

///////////////////

module testbench;

legacy_module_1 instace_a();

legacy_module_1 instace_b();

package pkg_a;

class cls_1;

    // cls_1 needs ability to call instace_*.task_x();

...classes in packages cannot contain hierarchical-references to anything outside of the package (so that rules out the easiest route.)

Next, I thought I would create an SV-interface, then declare extern tasks/functions inside the interface (i.e. 'proxy tasks', which then call instance_*.task_x() ) And create interface-ports on legacy_module_*, as hookup points.

But IUS 11 doesn't support it:

        extern task x;
                  |
ncvlog: *E,SVNIMP (ptclass2.sv,3|11): SystemVerilog construct not yet implemented:  extern tasks within an interface.

I also tried the alternate approach of 'importing' a task inside a modport declaration, but it seems IUS11 doesn't support that either.

I'm stumped...