Thanks, not something I use so was buried deep in the dark reaches of my mind. Gets scary down there...

While it's a Windows version, it's likely working the same way it does under Linux. Also, if that link is based on the user account, then Serviio won't see the link unless Serviio is changed to run under that user account. I can't find anything online that answers that.
I also see there's several switches when using mklink, so perhaps one of those might work better. I'm thinking specifically for /J switch, which creates a directory junction. I'm going to have to play with it and see what happens.