Inner Corset during meditation?

You do not need to use the inner corset while stacksitting; it is not a progression, no. While sitting, you should be able to relax completely, and when your bones are well aligned, they support that nice, upright, relaxed position. Inner Corset is only for challenging situations that would threaten the spine (discs, nerves etc) if you weren't using the Inner Corset muscles to support it and keep it long and decompressed. The most obvious examples of such situations are lifting, carrying, running and jumping. Yes, you can use the Inner Corset to lengthen the spine in preparation for meditation, or even as an exercise - engaging it periodically while you are Stacksitting. That certainly isn't harmful and can be a nice way to practice finding those muscles.