Can't get Qsys to generate a testbench for a custom component using exported signals

Your understanding/expectations all sound correct. I just tried it with one of my components with 13.1 Web Edition and it works fine basically as you wrote (new system, delete the default clk, add my component, export all interfaces including clk and reset, "Generate..." with Standard BFM's added).

For what it's worth, I do have some old components (ca. SOPC 9.1) which I cannot use the BFM's with (Qsys will produce errors, not the same as yours). The general issue is that my problematic components have incorrect or inconsistent interface parameter settings which work "OK" for synthesis generation, but cause problems for the BFM generation program.

My suggestion is that you review other Qsys messages prior to the error for possible indicators of the problems it might have with your component, or step back and create an empty "foo" component and populate it with your existing component TCL piece by piece until it breaks (or similar debug since you aren't getting a meaningful error message).