For SQL Server(or any other app) to access the remote file, you would need to have a drive$ share.

Let us say on machine m1, i have a file on d:\Folder1\mydata.bkp

if i need to access this data from another machine m2, I cannot use the UNC
\\m1\D:\Folder1\mydata.bkp unless there is a $ share on the d drive


Or if folder1 is shared, then i could use \\m1\folder1\mydata.bkp

I think your best bet is to temporarily share the folder, run the restor, then unshare the fodler.

Hope this helps.