At Tools -> Options -> General there is in the sixth line an option "Save files without prompting before build". If you check this POIDE save files before build, so the actual file in memory is compiled.
POCC, the compiler, is an external program so can not work with the POIDEs memory. If you really want to compile the memory without saving the source file, you have to write an AddIn, which saves the source files to temp files, compile them, rename them and link them. Perhaps you can take the AddIn for REL/DBG/PRF as template.