I do not know Domotich - I am working with openHAB through MQTT protocol, which simply reads the sent value of variable and does not wonder what type they are. Yes, I use S_CUSTOM variable to send RSSI value.
But it really doesn't matter what kind of variable you use. The variable type tells to MySensors that its value is for example: light level, temperature, or voltage, but you can send a temperature value using the S_POWER variable - no problem with that.
RSSI should be expressed as an integer, so what is realy important that you choose the variable type declared as integer value, such as i.e. S_LIGHT_LEVEL.
Once you have that value in the controller (openHAB), you can do whatever you want with it.
So it is in openHAB, and I hope in Domoticz the same.

@gohan I use V_VAR1 type of variable.

In the 2.2 version of MySensors (development branch) the new driver for RFM69 is built. It has new command for reading RSSI value: RFM69_getSendingRSSI() and RFM69_getReceivingRSSI(), so now is very easy to get RSSI value.
You can get some more information from ATC Signal Report which are useful - look at RFM69_RFM95_ATC_SignalReport example.

@Carywin OK, my gatewayMQTTclient with RFM69 via SoftSPI is working now. There was a problem in RFM69_new.cpp file with SPI_HAS_TRANSACTION variable which is defined in hard SPI.h but not in SoftSPI.
Now I test my gateway and I hope it will work without losing connection. Thank you once more for help.

@Carywin The problem with compiling by Arduino is with softSPI. When I switch it off my sketch is compiling properly. I have to implement softSPI library to new driver. Thanks.

@Carywin Is your gateway still working without losing connection ?
I have the same problem as yours: my gatewayMQTTclient with RFM69 by softSPI is losing connection with nodes. Sometimes it works by couple of days, sometimes only few hours.
I have tried to implement new RFM69 driver version of MySensors (2.2 dev), but arduino has not compiled my sketch according to errors in RFM69_new.cpp file (some variables are not defined). With old driver version sketch has compiled.
How have you run your gateway with MySensors 2.2 ?
Have you defined MY_RFM69_NEW_DRIVER parameter to work with new driver ?

It is possible to use RFM69 radio via SoftSPI which allow to build gateway with W5100 or ethernet shield. You need to change RFM69 driver and MyConfig.h configuration file.
I put modified these files at github repository: https://github.com/gieemek/RFM69_softSPI-for-MySensors-2.1.1
You can find instruction how to prepare gateway sketch at this repository too.
My gateway (as MQTT client) with ethernet shield and RFM69 radio via SoftSPI is working for 10 days without any problems.

I modified MyConfig.h file and RFM69 driver to allow you build gateway (ethernet or MQTT) with W5100 ethernet and RFM69 radio modules using SoftSPI bus. You can find it at: https://github.com/gieemek/RFM69_softSPI-for-MySensors-2.1.1

Modifications were easier than I expected, only that I am successfull (I'm not a programmer).
For now, the program did not switching between soft and hard SPI and I fail to realize it fast - really frequently change the file is uncomfortable.
I want to send my library to LowPowerLab to check it.
If you are interested in further modification of this library, I can send you by mail a list of changes I made.