I'll be reprogramming some of the TZ85a escs eventually, things are pretty busy at the moment, but they are in the pipeline

As a rule of thumb, it is usually escs that run atmel chips that are more easily programmable...
However as Nick says, some of these can be locked.
Occasionally locked escs can be force wiped, then have custom firmware flashed to them, but it can be a unreliable process with some risk involved to the longevity of the chip if done incorrectly...

Alternatively... some locked escs can have the chip removed and a new blank atmel mcu soldered in and flashed with the custom firmware, but you must be proficient in soldering to achieve this.

I usually use a needle point tip on a temp controlled antex iron, with either super fine solder, or solder paste.
Focused heat flashing can be used, but I prefer to see every joint formed individually =)

Currently looking into other escs that could be converted... looking for something cheap but reliable... or at least with some potential to inflict moddige upon