I wouldn't worry much about being refillable. The fluid will last as long as the pump for most coolers. There are a number of Cooler Master cases called, "cosmos," so I am not sure what size cooler will fit. However, I would suggest using at least a 240mm cooler, 360mm would be best since you are interested in overclocking. If you really want to be able to refill it, I believe the Swiftech Drive X3 has a fill / drain port. Others like the Fractal Celsius and AlphaCool have user serviceable fittings, but that doesn't necessarily mean that you can refill them without modification.
You could also get a custom loop kit from a brand like EK or XSPC. Be aware that copper and aluminum water cooling components are incompatible, most closed loop or AIO style coolers use aluminum but there are exceptions. Likewise most custom loop's use copper, but there are also exceptions there (notably EK's Fluid Gaming line).
