In the last episode (Oct 05), Martin Baehr said:
it could just as well change any argument in the array.
does it make sense to allow changing individual arguments? (except argv[0])
Depends on what you're trying to accomplish. If you are rewriting argc/argv to make it easier for later parts of a program to parse (a la getopt), then you can simply change argv directly. If you are trying to change what the ps command prints, each OS has a different method, since many kernels cache the argv string so ps doesn't have to swap processes into memory just to print the commandline. The BSDs have a setproctitle function that does the dirty work for you, and you can take a look at the sendmail source for all the other methods.
src/README describes the different SPT_TYPE values, and src/conf.c has the implementation.