The native Transform Points always rotates from the center of the selection. There's no way to set an arbitrary origin like you can with a layer.
However...
DKW Root created a Transform Points mod that let's you set a different origin for the selection and transform it from that origin. You can get the mod from here:
viewtopic.php?f=12&t=30487
If you really need this capability, I highly recommend the mod. You set the selection origin by Shift-Alt-clicking and then just transform the selection with the tool as normal. If you need to use the normal center selection, just click the Center button.
The 'downside' is that you need to click the Center button every time you want the use the tool the 'normal' way, which can get annoying if that's the behavior you expect most of the time. Depending on what I'm doing, I sometimes have to swap the tools. (Sometimes I think I should try to mod the mod to load as a separate tool but just too lazy I guess.)
