I use the above process on my server; I have the main instance of M32 that filters all the outgoing mail.
If the "From" header = a.com, message is processed by the main instance.
If the "From" header = b.com, message is moved to the mbox b-out, where wsmtpex gets it, and forward it to the "B" instance of M32.
If the "From" header = c.com, message is moved to the mbox c-out, where wsmtpex gets it, and forward it to the "C" instance of M32.
Unfortunately there is a couple of cons (at least, here)
1) when the msg is moved to the b-out or c-out, the "From" header is removed.
2) it seems that wsmtpex does not support multiple recipients; so, if I send an email to - say - myfriend1@gmail.com and myfriend2@gmail.com, only the last receives it, because all the other addresses are lost.
It may be a misconfiguration, but I couldn't get rid of this.
Best regards
Filippo