Can I set fsSelection bit 5 and 6?

So, that’s a common problem: we create a font family that includes light styles, and if it is used in MS Office, the user clicks the bold button, which creates a faux bold, ugh. Of course, we’d like to prevent this.

A nice trick I have been playing around with for many years is to simply set fsSelection bit 5 (BOLD) to 1, and bit 6 (REGULAR) to 0. See OS/2 - OS/2 and Windows metrics table (OpenType 1.9) - Typography | Microsoft Docs
This prevents MS Office from creating a faux bold even if the font is not style-linked to any “regular”. Although it’s slightly hacky, there doesn’t seem to be any downside (please correct me if I am wrong).

Is there a custom parameter that allows me to do this in Glyphs?

Not yet.