You can put two entries in your External Data Source Box. Make one the Windows path you've already posted and then add a second line with the Mac path:
I realize that I can put more than one entry into the External Data Source Box, the problem is that in order for
to work, every single mac would have to have the same volume name (Which is something I can't control). So is there a way to do it without having to resort to a line for each and every different Mac that needs to connect to the network?
And on a slightly related note; Is it possible for a script to change the data in the External Data Source box?
No and No.
You will have to get all your client machines set up consistently. (Or you'll have to list quite a few different variations in your data source box.)