Use at your own risk. Experimenting with ModifyUserData is dangerous. Backup your user_data.json.lzo ...
Thank you

The JSON looks solid, and validates properliy - however I'm having trouble with my request...
Trying:
http://ip:3480/data_request?id=lu_action&DeviceNum=56&scene=1&action=ModifyUserData&inUserData={"scenes":[{"name":"HeatON","Device_Num_56":{"event_2_1":{"argument_1":"25"}}}]}
Yields: "error: no service/action"
Of course, I'm not stating any service ID's like my colleague did in his example:
http://ip:3480/data_request?id=lu_action&DeviceNum=0&serviceId=urn:micasaverde-com:serviceId:HomeAutomationGateway1&action=ModifyUserData&inUserData=JSON_CODE_GOES_HERE
But then again - what serviceID am I supposed to give? I can't find any useful serviceID when looking in UI4, and I'm not sure where to look, in the user_data or lu_sdata output. user_data is 5500+ lines big, so trying to get an overview of all the objects seems a bit futile. I've grepped the file for serviceID's and some other keywords I thought relevant, but to no avail...

The documentation of the LuuP actions and the parameters they take are somewhat sparse..
EDIT:
Do you want to modify it dynamically, based on a certain event?
If not, why don't you edit the event and modify that value by hand?
I wanna be able to modify the events remotely.
Maybe I should give some background info

I work for a small company specialized in home automation, developing software for the Vera among other things. The above mentioned scenario is a real world Use Case: One of our customers wants to control the thermostat in a set of houses. I wanna make a tool/script to automate that process, so the customer doesn't have to manually log into all the Vera boxes and configure the scenes by hand.