Of course as usual I expect that Synchron Strings II will include kind of a complete Sampleset with Sordino. For Now I did the nearly same:
1) avoid as much as possible scores demanding sordinoi-Passages, and
2) if there still some not avoidable I use not LPF6 but LPF12 completly open for non sordino and completly closed for the sordino effect.
You can hear the result in the Tschaikowsky Serenade at the Ending of the III. Movement where the strings repeat the slow chordmelody from the start of the movement but this time muted. the slow Beginning of the IV.Movement is likewise LPF12-muted you can hear the difference, when the high tempo of the movement starts.
I confess it is not as beautiful as real sordino-samples are, but it is more than nothing (and comparable to many other Libraries which only provicde that "Filtertrick" or even worser only Sordino-sustains) I hope that helps at least a bit for the moment. Otherwise you probably have to wait for further Synchronstring releases.