Hey Leo,
welcome to the forum!
I don’t think any one of us here is still an expert on the Matlab interface (and it will get removed in a future update, so a migration to the Python interface is highly recommended) but I’ll do my best.
I think the SingleGRM as described here is just a convenience wrapper around the General Rate Model. So we can treat it like a regular GRM, which does not require a bound state for each component. So you can set the bound state for salt to 0.
Let me know if that works or doesn’t work.