ptrace and system call interception

I'm looking into porting the user-mode kernel port (see
http://user-mode-linux.sourceforge.net) to IA64. This depends heavily on ptrace's system call interception and modification abilities.

I need to be able to annull system calls by changing the system call number (I use __NR_getpid). My reading of entry.S, and running some test code, says that this doesn't work. Can this be added without too much difficulty? If so, can sourceforge (or intel - whoever owns the protos available through sourceforge) be persuaded to update without too much difficulty :-)

Another thing that's not obvious is where the system call arguments are stored. I can locate the system call number (PT_R15) and its return value (PT_R8) without too much difficulty, but I don't see the arguments.

Thanks, Jeff