Personally I would use Forth for this. PicoC looks interesting but if I were to weigh my options (i.e. PicoC vs Forth in his example app UAV-onboard system, I would definitely go Forth).
As I understand, the main appeal of using C as a scripting language is the fact that you use the same language in compiled and interpreted part of your application.
Drawbacks of using C interpreter for scripting could include large footprint and external dependencies, but as far as I can understand, they are not the case here.
In the case of embedded programming, Forth is often made for the job. C is great for embedded programming too, and you can always mix the two languages. C isn't made for scripting, and hacks like PicoC don't appeal to me since it will always remain a hack.
While not exactly a scripting language per se, Forth does have amazing interactive capabilities. It's extremely light-weight and it's easy to implement a new Forth on a new hardware architecture. Footprint issues aside, development time in Forth is usually shorter, and the ability to connect directly to a device's Forth "shell" and try out Forth words (functions), create new words, play around with hardware in real-time is invaluable.
Forth code is less readable if you don't know Forth, and occasionally Forth programmers write ugly code, but that is definitely the case for embedded C programmers too.
As I understand, the main appeal of using C as a scripting language is the fact that you use the same language in compiled and interpreted part of your application.
Drawbacks of using C interpreter for scripting could include large footprint and external dependencies, but as far as I can understand, they are not the case here.
So what is your reasoning for Forth?