Customised OpenWRT Firmware to interact with external captive portal

General information for the business: OpenWRT wifi router interacting with external captive portal
Description of requirements/functionality: I need someone to customise a copy of OpenWRT so that it can interact with a external web based Captive Portal.

NOTE: The hardware (Access Point) running OpenWRT will be a Raspberry PI 2 Model B using a USB WiFi dongle

I will provide a full technical specification of what is required however here is a general breakdown:
* Router will use OpenWRT

* User redirected to external Captive Portal when connecting to SSID

* When Captive Portal authenticates (managed server side) the user will be redirected to 'http://captive.success' which will be programmed into the routers DNS to then allow the user to access the internet

* Every 15 minutes the router scans which devices are connected and sends it to a specific URL, if the URL responds with a MAC address it needs to re-authenticate

* Reverse SSH needs to be added so the device can be pinged and accessed remotely, also so updates can be sent

I will also need a list of where each customisation sits so it can be changed in the future.
for example: Change the Captive Portal URL = Line 2299 in hostpd.conf etc...

please note: No work is required on the web server, only for the firmware. The authentication will be handled server side and will simply redirect to the URL which will enable access if they pass the authentication.

Thanks
Specific technologies required: OpenWRT, Linux
OS requirements: Linux
Extra notes: Upon proposals, technical specifications will be sent, please let me know if you have any questions