As I said in the other thread, it's in the 
Wiki.
	
	
	
		Code:
	
	
		  int Uppermemfd;
  unsigned char* UpperMem;
  Uppermemfd = open("/dev/mem", O_RDWR); //Open the memory for reading/writing
  UpperMem = (unsigned char*)mmap(0, 0x2000000, PROT_READ|PROT_WRITE, MAP_SHARED, Uppermemfd, 0x2000000); //mmap the upper 32MB.
	 
 
This gives you a pointer (UpperMem) to use to access the upper 32MB.  It doesn't extend the heap size or give SDL any extra memory to use, you can do what you like with it. 
You have to manage the memory yourself - the  
Wiki includes functions for providing a malloc function.
P.S.  did I mention the 
Wiki?  
 
P.P.S. This is not the MMU hack, just a way of accessing the upper 32MB of memory, which is what I believe you were looking for.