I am in the same boat, where I don't want to eat up my internal HD space but still want to have some redundancy for my Google Drive account.
I am pretty sure I have found a working solution. I'll also run down the things I have tried that didn't work so that people can be spared the time to try these things.
I have a Drobo and have some shares mapped as windows drive letters. Several months back, I set the share permissions to "Everyone" on the Drobo dashboard, and I was able to set that share's mapped drive as the Google Drive root folder in the past.
HOWEVER, something happened with an update somewhere and it stopped working. I kept getting the message that the folder is not writable. I was clearly able to write to the folder through explorer. I tried connecting to the share as Administrator and then running sync as an administrator. That didn't work either, so I concluded that it wasn't a permissions problem, but perhaps Google updated the sync program to flat out reject UNC paths. But I tried some permissions tweaks anyways...
I tried going into the folder properties through explorer and changing the ownership, and setting permissions to full control. Windows couldn't do it.
I tried changing the credentials that windows uses to map the drive. Nothing worked there. I tried using different ways to try and fool the sync program into thinking it was not a UNC path. Nothing worked there. (subst, mklink) It kept seeing the UNC path through the mask.
I tried creating a VHDX on the share, and then mapping it to a folder on my C drive. That seemed to work but Sync kept crashing. ( \drobo\share\GDrive.VHDX -> c:\GDrive\ )
Then I tried mapping the VHDX directly to a drive letter. That seemed to work, but sync was getting hung up on the System Volume Information and Recycle folders. ( \drobo\share\GDrive.VHDX -> G:\ )
SO... I created a folder, on the root of the VHDX that is mapped to a drive letter. I set that folder to the Sync folder. It has now synced 3249 files and 2GB without a hiccup. I have a TB of data to download, so (fingers crossed) I am feeling confident that this solution works. I will update the answer if I discover any issues with this. ( \drobo\share\GDrive.VHDX -> G:\ ) THEN created a GDrive folder on G:\ and then selected G:\GDrive\ in the Sync application.
HTH
Edited to add: I also tried InSync, but for a terabyte of data in 100's of thousands of small files, downloading 2 files at a time would take WEEKS.
I also tried ExpanDrive, and although I was able to map a drive letter directly to Google Drive, it was a very slow connection and still left me with the need to set up some sort of sync between the mapped cloud and my NAS. When I tried using CrashPlan to back up from the mapped Cloud, it couldn't see the drive letter that ExpanDrive had mounted. So I gave up on that.
As an added bonus, mapping the VHDX to a drive letter makes thumbnails work in explorer even though they are in a network location. Woohoo!