This is probably something *so* obvious to most people who use solaris all the time, but I had previously never joined the dots.
The reboot command can pass arguments to the boot program and kernel on restart. If the boot arguments start with a hypen you need to include a delimiter of — (two hyphens). You also may need to quote the command if whitespace is important.
So if you want to issue a boot -r command you could do the 2 step process of bringing the system down to the OBP and then issuing boot -r or you could just
reboot -- -r
To reboot and install solaris from the network
reboot -- 'net:dhcp - install'