Do it now... what case do you have? That will help me pick what to link for a drain port.

The simple, non-20-questions answer is a T, plug, and 2 more fittings.
Since you are going with the XSPC kit, you could keep it all XSPC:
1x http://www.frozencpu.com/products/20265/ex-tub-1838/XSPC_G14_3-Way_T_Adapter_-_Black_Chrome.html?tl=g59c669s2182 (out of stock right now)
2x http://www.frozencpu.com/products/22572/ex-tub-2543/XSPC_G14_Thread_716_ID_x_58_OD_Low_Profile_Compression_Fitting_-_Black_Chrome_V2.html?tl=g30c409s1203) - screwed directly into the straight portions of the T
1x http://www.frozencpu.com/products/13539/ex-tub-943/XSPC_M20_to_G14_Fillport_-_Black_Chrome.html?tl=g59c669s2146 - screwed directly into the angled portion of the T.
 
Put this whole assembly on a low portion of your loop (or a portion that can be made the low portion if you turn/tip the case), and when you want to drain, stick a tub under the plug and release the plug.  Voila - loop drains!
 
~Fus
 
EDIT: put in the V1 compression fittings originally.  Finally looked at the kit, and saw the V2 fittings, so changed recommendation to match.
EDIT 2: Alternatively, get the port and 2 fittings from above, but instead attach to the spare port on the Res using a short bit of tubing.  Voila - drain from the Res.