For now you'll have to take the approach briefly mentioned at the bottom of Package Operations - namely adding an EXE package and setting the command-line to the associated executable. In this case I believe the install operation's command-line would end up being something like:
"[SystemFolder]wusa.exe" "[ISPREREQDIR]your.msu"
(I'm a little confused whether there needs to be an additional backslash after [ISPREREQDIR], and you may want to look at the other command-line options offered by wusa.exe, such as /norestart - see wusa /?)
