There are ways to do this, but none of them are cheap. One of my friends has a TS-E 17 and 14/2.8 and uses a Cokin-Z Pro filter holder with it. I believe he uses a flash bracket to hold the filter holder in front of the lens. Another solution is the Lee SW150. It's actually designed for the Nikon 14-24 but as I understand it is not difficult to get it to work with the TS-E 17 or 14/2.8. Of course, given the state of things for Lee it will likely take some time to get ahold of one.
The big issue cost wise is you need 150x150 filters for this to work (100x150 for Cokin-Z). I have a TS-E 17 but all of my filters are 84x120 and I am not about to buy a complete set of new ones. The 150x150 filters are over double the price ($225 vs $99 for a SR 2 or 3 stop GR hard or soft grad).
Personally, I just accept the fact that the TS-E 17 does not take filters and use my 16-35 or TS-E 24 II when I need them.