I haven’t had the time to test your work out earlier, but I got it working quite fast.
My setup:
SmartThings V2 UK hub
Domoticz V2.3213 running on Raspberry Pi B (type 1)
RFXCOM - RFXtrx433 USB 433.92MHz Transceiver
Somfy Blinds (4 in total)
I also have a busload of Philips Hue and Living Whites devices (bulbs and on/off switches), but I decided not to assign them yet because I am planning to reset my whole home setup because I am having weird problems on the Philips side, no use making it more complicated 
So I have selected only the ‘Blinds’ type.
In the “Custom HTTP/Action:” I have added in FIELD#1:
https://graph-eu01-euwest1.api.smartthings.com/api/smartapps/installations/(code from URL of SmartApp when editing it)/EventDomoticz?access_token=(OAuth Client ID from App Settings)&message=#MESSAGE
(this is the UK server)
Because I have a password set up on my Domoticz server, I needed to whitelist my local network to work without authentication, in Domoticz->Settings->System add to “Local Networks (no username/password)” a wildcard for your own local network (for example “192.168.1.*”).
Then I went to the Switches menu in Domoticz, selected the blind I wanted to control, clicked ‘Notifications’ and added the 'Switch On" and “Switch Off” notifications for “http” only.
After that searching for connected devices gave me a list of my 4 blinds, and I can control them just as expected.
The only thing that doesn’t seem to work is the status notifications of the blinds. I am only seeing the status change when I control the blinds from within ST, but when I open/close them through Domoticz the status in ST stays the same.
The ST app does not reflect the current status (open/closed/stopped), even though I can see in the ST logfiles when I refresh the blind the correct status:
"LastUpdate" : "2015-11-06 19:22:02",
"Level" : 0,
"LevelInt" : 0,
"MaxDimLevel" : 0,
"Name" : "Rolluik studeerkamer",
"Notifications" : "true",
"PlanID" : "0",
"PlanIDs" : [ 0 ],
"Protected" : false,
"ShowNotifications" : true,
"SignalLevel" : 7,
"Status" : "Closed",
So even though it says “Closed” here, in the ST app, the current status remains “STOPPED”.
edit:
When I am looking through the domoticzBlinds device type, I see references to “switch” instead of “blinds” in the event parsing part, could it be that this needs to be changed? And in the Domoticz Server in the last few lines I see references to the US server. Should we change this to the UK server when using the UK V2 hub?
And one more edit: could this be affecting other UK hub owners with this project as well?