I added side buttons to an WMO and wired them + L/R buttons to the PCB of a Coolermaster Inferno, which has programmable debounce time that goes as low as 250μs. Apologies for phone camera quality. I'm not a camera enthusiast.
The side button array and connection to external PCB were done using microUSB cables. You can draw voltage for all switches from a single node on the WMO PCB while having the perfect number of pinouts on USB to send 4 button circuits to the external PCB.
The microUSB jacks are taped to the rear LED. Was originally going to epoxy them to the PCB but decided against that in case I needed to repair any of the connections. Eventually I will find another thin microUSB cable to replace the WMO stock cable as well.
Buttons and switches were ripped from the Inferno and epoxied to the WMO shell. The little cage thing makes a convenient platform for mounting switches. A few dabs of hot glue were used to create protrusions on the buttons for switch actuation.
I used a spare USB port from an old motherboard for a quick connect interface with the external PCB. Doing everything with USB is handy as the colour coding of wires remains consistent throughout every stage of wiring. When in use, I keep the external PCB in a case. Having the extra cable doesn't feel heavier if positioned well, and replacing the stock WMO cable with another thin microUSB cable will probably end up feeling lighter than a single stock cable.
I would've used an IMO 1.1 and done the wiring only (for the low click latency) and not fabricate my own button assembly but none were available locally. Eventually I'll find one on craigslist/kijiji and mod that out too.