How have you managed “Strange Methods” and “Artificer” in Flashbacks?

How have you managed “Strange Methods” and “Artificer” in Flashbacks? Did you charge Stress using the Magnitude rules? Called action rolls to enstablish the usefulness of the object? Something else?

Last session I had my Whisper pay Stress to use a one-shot electroplasmic freezing device: it was cool (pun intended), but it seemed to overlap too much with the Tempest ability.

Any suggestions/ideas?