Creating custom flashable partition on Android

Question

I'm working on a project that utilizes Android to power the device. In my case that's a tablet that uses standard MTK processor, and since the company that I work for produces a tablet by themselves, I have an access to a full firmware for it (device drivers, radio drivers, system, etc). However, I am trying to create a custom partition that would store some data, but should be flashable via flash tool. After searching for the last two days, including https://source.android.com/devices/bootloader/partitions-images I only found that I can create the partition via adb. That's not what I want. I want to be able to create something like /MISC partiton that could be flashed via flash tool. Is it even possible to do something like this, and how? Any advice is welcome.