Here is what I have found as a successful way to export nulls and cameras to After effects and then connect Particular to the nulls.
Using Adobe After effects live link in maya 2017 Update 4
I am not entirely sure what the rule is for exporting nulls but I have curve in Maya and like 8 nulls parented to it. I have animation on the curve and none on the nulls but I am pretty sure you can animate the nulls as well.
1. Open Ae Live Link window in maya
2. Open After Effects and create a comp that is the same size as your rendered Scene in maya. Import your movie and put it in the comp. Then under Window, Extensions open Autodesk Maya Live link. It should connect to Maya when Opened.
3. In Maya, Select the camera and the Locators and middle mouse drag them into the Ae Live Link window and hit the little paper plane send button.
Note: At one point I used the Export Button in the Maya AE Live link but it does not seem to be a requirement. I could be wrong but i thing the export and import in the live link are there in case the two don't connect so its basically the manual way.
4. In Maya Check that everything is moving as it should.
Note:when I had my animated curve parented to another object I was getting strange results so my hierarchy in maya is as fallows.
Curve with all of my locators as children
5. Now that all of your cameras and Locators are in Maya look at one of the nulls position. The numbers are gigantic. Particular cannot be seen if you attach it to a null because the "far vanishing" number cannot go beyond 1,000. So How do you fix this. Smash head on desk for a while.
6. Create a 3D null and then select all of your Locators(now nulls) and camera and parent them all to your new 3D null. Now scale your 3d null down to like 3% and then unParent the nulls and check the number on the position. if your values are not under 100,000 then rinse and repeat. Scale the new 3d null Back up to 100 percent parent everything. Scale Down to 3% and then Unparrent everything and check the numbers.
7. Now Create a solid with particular and quckpick the Emitter position x, y to one of the locators of your choice and then quickpick the Emitters z position to the z of the same locator and you should be in business. I hope this helps. If you find a better way to export out of my with smaller numbers please let me know!
Cheers and God Bless!