You can write to an unwritable partition using this simple syntax. Many security engineers strongly encourage SysAds to lock down (read only) their/usr partitions. However, this can present problems during upgrades. Here's a quick and dirty workaround.
#mount -F ufs -o rw,remount /usr
Frankly speaking, I've only been able to remount /usr to write. I've always had to reboot the system for read only to take effect. Suggestions?