Thank you both for your input!
@Glen_King, I’ve been looking at using both IFTTT or MQTT as alternatives. However this involves too much “user configuration” (explained below), I think. I honestly haven’t experimented with this myself yet, since I’m currently researching options before I go ahead and send a purchase order in order to start playing around with hardware.
@JDRoberts the microlocation tag was spot on. I spent all afternoon reading what others in the community have been doing. It does look like some have been trying something quite similar to what I’m trying to achieve. However, perhaps a little clarification on my intentions would be in good order.
I’m working on a project that focuses on behavioural analysis of patients. It’s based on monitoring daily activities and patterns, which are further analyzed in my backend. The activity tracking (read, assumptions on activity based on the location of the person in his/her house) is only a part of a more complex set of imputs to analysis algorithms. In fact, a current implementation exists using z-wave motion sensors and door/window sensors, configured on a custom gateway (controller) which reports any activity on them to the server. In the server, the algorithm takes care of filtering false positives, multiple users, and eventually handling pets and other cool stuff.
It all works fine and all, except for the part where it doesn’t make sense to install such sensors in homes where there’s a pre-existing home automation / surveilance network. It would make more sense to use those existing sensors (without altering their original state/functionality) and just use their output for the behaviour anaysis as well.
Since the goal is not something HA related, I’m not sure if this has been done before. I drew a diagram that possibly explains the ideal setup:
The idea would be to keep the HA/Security network “intact” (i.e. keeping the smart home functionality or whatever those sensors and lights were being used for), and just “extract” the sensor information somehow. The existing network is generic, in the sense that I cannot know if they are using a SmartThings HUB, Wink 2, Vera, etc. Therefore solutions involving configuring IFTTT and such would involve too much micromanagement. If this was going to be done once, in one location with one known HUB, that would be my go to solution for sure.
I have thought of two different approaches, but I would appreciate input from the community who has more experience on this. I have hopes that if this project succeeds, the outcoume would certainly benefit the HA community as well! 
The first approach I have in mind is depicted in the image above. Would it be possible to bring in say, a ST, or a HASS RPi or similar into a location and make it the primary controller while keeping functionality on the secondary? This new primary controller should be able to communicate with the server and send activity from the original sensors, as well as optional additional sensors (for example, the house didn’t have enough coverage with the current set of sensors). If this controller could be for example a ST and use IFTTT or an MQTT broker for example it wouldn’t be a problem, as long as it could be “pre-configured” somehow.
A second approach would be developing a device (hardware) that connects to the network as a device (Zigbee), and can somehow listen to the traffic (sort of like a sniffer, but that the user voluntarily adds to his network). I haven’t read much about people achieving this.
Local would be preferred over cloud, but it wouldn’t matter much as long as the objective is achieved.
Any input would be appreciated! I’ll report back any new findings I come up with as well. 