This comes a little late but yes.
I’am not using currently RAK’s concentrator modules but I believe almost all SX1303 concentrator modules have a built-in Fine timestamp feature.
Actually Fine timestamping can be used if concentrator module has a SX1303 chip inside and has a wired PPS line to the GPS. It seems to vary which device vendors have implemented this Fine Stamping feature on their platforms.
Fine timestamping property tested with following concentratord/module combinations:
- n-fuse:
Concentrator Card LRWCC3-MPCIE for LoRaWAN (external GPS/PPS module attached) - Heltec
HT1303 + LR1302 868 LoRaWAN Hat, (This is really inexpensive and good alternative;)
Here are a couple of stories from my own point of view on the subject.
My intention is to publish all this knowledge on the subject as soon as I get the implementations ready for publication. And this phase just seems to last a long time 
I am currently working with the idea of 3D positioning of sensors (without gps) based on this fine timestamping and TDoA or RSSI multilateral positioning method…
At the end, here is a litle gem: ChirpStack-concentratord already has the fine stamping property implemented inside.