Is there any way to change the LoRa duty cycle via the firmware code such that devices send payloads at a rate other than once every 15 minutes (as is currently defined by the LoRaWAN code)?
Duty cycle refers to a percentage of total air time, which the device may remain on air. This would be required by local RF regulations. You probably cannot legally modify this, if you really meant to modify the duty cycle.
On the other hand, if you meant that you wanted to change how often the device reported data, then you could do that if the device allows you to change its reporting intervals. Do check its documentation, to see whether it has such a feature.
If the reporting interval is hardcoded into the firmware, then you need to change the firmware - which depends on whether you have its source code. But given that you had to ask, I got this feeling you don’t really have the means to do this.