You can create your own version of malloc() and place that code in the
link list before the standard libraries, then your version will simply
override the standard implementation. You would also need to override
free(), even if it were just a dummy function.

I did exactly this for example when I needed to integrate zlib
decompression in a bootloader that could not use the standard library at
all. zlib called malloc exactly once with a predictable size, so I just
dummied a malloc().

All that said, why can you not simply use the standard malloc() function
(you only have to implement __sbrk or sbrk_r to make it work, examples
are provided with the WinARM tools in the syscalls.c file)