How about making the result from get_temporary_directory() available in user land, most likely as a constant named TEMPORARY_DIRECTORY  This would be useful in that we'd know where this is, in both Windows and *nix.  The code that defines the internal variable temporary_directory is here:

http://lxr.php.net/source/php4/main/php_open_temporary_file.c#164

I'm not a devel guy, but the following hack seems mildly appropriate:

REGISTER_STRING_CONSTANT("TEMPORARY_DIRECTORY", get_temporary_directory(), strlen(get_temporary_directory()), 0);

Not sure where to put it though, or if it's fully correct (doubtful), but please consider this idea as it would be nice.

One *possible* concern is security but I think it's worth it, *maybe* disable this option in safe_mode.  It's not like the TEMP directory is a big secret, nor is viewing various related directives like session.save_path and upload_tmp_dir.