hey actually you can do this.
that is how i shoot.
Im in AV mode
in the custom function mention you can decouple the focus and exposure from the shutter button, such that AF is only triggered by the AF-On button. Once you focus, you can move the center AF point to wahtever you want to expose for, then half press, then re-compose.
OR (which is the more inuitave way) is to use the * for what you want to expose for, then focus on your object with AF-ON, then pressure the shutter...note you have to do this within a certain time other wise the exposure lock with reset itself
if you need help with this let me know//its getting late