The simplest (and easiest) solution would be a ESP8266 with two contacts running Tasmota. If you configure the contacts as switches you will get a near instantaneous MQTT message to openHAB.
ESP8266 will/can be a solution, but no experiance how to program these (naked) boards.
What about the sonoff basic parts (and yes, they have inside EPS8266 I think), as I saw now they also have 4 GPIO’s inside.
I also flash my first to Tasmota including MQTT …
Can I use two of these GPIOs for my catflap-contacts, and two to connect two LED’s (status, if cat is in (green) or out (red)) ?
(I still have these LED’s on my Raspberry, connected on two GPIO’s there)
I recommend buying an ESP8266 development board, it comes with header pins, resistors, diodes,etc… installed on the board. A USB cable is used to program and power the unit.
If you’ve flashed tasmota to a Sonoff device, then flashing firmware to an ESP is done the same way, except you use the USB cable. Personally, I like to use Esp Easy firmware on all my ESP devices but you can use what you like.
I have a mixture of Sonoffs and ESP8266 IoT relays. The sonoffs are good but, increasingly, you need to solder a PCB header to the board whereas a Nodemcu/Wemos board can be programmed and powered by USB. You could probably use a Sonoff Dual for your application
I only have 230V in the near, so USB power will be not an advantage …
But still unshure if the four provided GPIO’s from the “sonoff basic” can be used for this case:
2x as output to connect two LEDs
2x as input for the two contacts
(the relay function will be gone, no problem. And connect cable on the plumb or circuit board will also not a problem!)
Search google, you may find a tutorial for hacking the Sonoff to add addition GPIO pins.
Now I order the Device WADWAZ-1. In germany it’ s selled i.e. from Schwaiger, called “ZHS09” (Type-ID: 2001/0106). This is also labeled as a zwave PLUS -device.
Scott give me (great!) support here to bring this device to work with two contacts.
I will provide a short “how to” if we finished, hopefully soon and successfully.
A little more detail… the Vision ZD2102-5 does have contacts for an external switch. The manual provides the details. After some testing, we’ll get the device db updated.
After few testings it seems that the external contact from the “Schwaiger ZHS09” is connected to the reed contacts! Also Scott is thinking now so.
So not independently as it looks! So not usable 
The Schwaiger is a “zwave plus” device, and can it be that all zwave plus devices has no second contact?
Maybe one can give me a hint what battery powered device (which is available in germany, ie. amazon.de) has a “real” second contact.