If it's individual files you're handling on the server side, then a IOKit driver is the wrong way, as that's only good to handle block level drives, I believe. Instead, you'll have to implement a VFS (Virtual File System). Sadly, there is little documentation on that. And it's a lot of work, many weeks.
A few pointers from someone who hasn't even accomplished that yet:
- Read about VFS in old BSD documentations. You may even have to purchase a book on paper for that.
- Read the code, i.e. get the Darwin source and look for any functions starting with "vfs_".
- Look for open source VFS OSX implementations and study them.
And be aware that since OSX 10.10 (Yosemite) you will need to have a special signing "KEXT" certificate from Apple, for which in turn you will need to buy the yearly membership. And if you have that, you may as well send a tech support request to Apple asking for pointers on this topic.
Good luck!
Thomas