I am not familiar with those devices, but they would need to be quite robust to deal with the OCV and amperage. What You are describing is what the output end of an inverter welder is doing. If used with a single phase transformer/rectifier there would be some variation in the output wave form from input frequency ripple, less so with 3 phase input. Due to the typical DC welder transformer operating at line frequency, the efficiency would still be low, unlike an inverter welder. Stainless TIG as I know it at least is a DC process, so wave form is not an issue.