Why don’t you look at vmware and vmware convertor. You can download the free esxi3 and install on a box. Then use the vmware convertor to convert your physical machine to a virtual. Then take the phyical server offine and boot the virtual and do whatever you need to do on the physical server offline?
Or you could just perfom what you are proposing out of hours? Remmeber to takew full backups before you commence work tho!