hi,
there is no way to "upload" to the bootloader area using the API calls
Why? Datasheet I have, does not say so. It says, that FLASH memory contains 64kb and this memory is both parallel and serial In-System Programmable (ISP). It says also that there are three ways to program all the flash memory:
- ISP via UART (bootloader);
- API (quotation: the FLASH may be programmed or erased in the end-user application by calling low-level routines through a common entry point in the Boot loader.);
- the parallel method by programmer.

And finally what it says about BLLB:
BLLB - Boot loader Lock Bit
Set to allow programming and writing of the boot loader segment. Clear to forbid software programming and writing of the boot loader segment (default). This protection protect only ISP or IAP access; protection through parallel access is done globally by the lock bits LB2-0.

So, what is a problem? User software must set BLLB and then modify bootblock area via IAP. Indeed, it should be done careful because IAP may corrupt/modify itself.

Regards,
Oleg