So far, the best shot is to declare variables within the mixin and use them outside:

.test-mixin(@varname)
{
    test-prop: @varname; // Do something.

    @res-1: 7;
    @res-2: 8;
}

:root
{
    @test-var: 5;
    @result: .test-mixin(@test-var);
    --test-1: @result[@res-1]; // 7
    --test-2: @result[@res-2]; // 8
}