FreeRTOS (and most RTOSes for that matter) do not work like general purpose operating systems (GPOS), they are not generally designed to dynamically load and execute arbitrary user supplied applications. In most case you use an RTOS because you require hard real-time response and the execution of third-party code could compromise that.
Most RTOSes (FreeRTOS included) are no more that static-link libraries, where your entire embedded application is statically linked with the RTOS and executes as a single multi-threaded program.
Again many RTOSes (like FreeRTOS) are not operating systems in the same sense as a GPOS such as Linux. Typically the RTOS services available are the real-time scheduler, inter-process communication (IPC), thread-synchronisation, and timers. Middle-ware such as a file system, and network stack for example are either optional extensions or must be integrated from third-party code.
One problem you will have with FreeRTOS trying to achieve your aim is that a "task" is analogous to a "thread" rather than a "process" in the sense of a GPOS process model. A task typically operates in the same memory space as other tasks with no memory protection between tasks. Tasks are not separate programs, but threads within a single application.
If your target has no MMU then memory protection may be limited in any case, but you may still want third-party applications to be conceptually independent from the OS. If your processor does not have an MMU, then running arbitrary third-party dynamically loaded code may be a problem for system integrity, safety and security. Even with an MMU a simple RTOS kernel such as FreeRTOS won't use it.
Operating systems with real-time scheduling that can load and run application code dynamically as separate processes include:
Also VxWorks has the ability to load partially linked object code and dynamically link it to the already loaded code. This is not the same at a process model, but is more akin to a dynamic-link library. What makes it worth mentioning in this context is that the VxWorks shell can invoke any function with external linkage by name. So you can load an object file implementing a function and then run that function. You could in principle implement the same functionality on FreeRTOS, but it is non trivial. The shell is one thing, but dynamic loading and linking requires the application symbol table to be target resident.
If you don't need hard real-time (or your real-time requirements are "soft") and your target has sufficient resources, you may be better served deploying Linux or uClinux which are increasingly used in embedded systems.
If the code your end-users need to run are tightly related to the purpose of your device rather than "general purpose" in nature then another possibility for allowing end-users to run code is to integrate a scripting language interpreter such as Lua. In this case you would simply load the script from a file system and pass it to the script interpreter. For more general purpose requirements a Java VM may be a possibility.
