Well, it depends exactly what you want to modify.
The main "listener" class, which is what processes the incoming bug reports is defined on the main config, so you can point it to your own class and just make sure to inherit the base class and keep the method signatures.
In addition there is also the rules engine which lets you create your own rules for message processing. Each rule just have to inherit from bugLog.components.baseRule
If what you want to modify is the actual "HQ" web interface, then things can be more complicated. However the entire HQ app is pretty much a front end to bugLog.components.hq.appService, so you can build your own interface around that. Although that may be more than what you have in mind.
Oscar