I use velcro to hold my res. to the outside of my case and to secure my pump to the inside floor of my case.
Sounds like it would work just fine in the situation that you are describing. Just make sure that you secure the velcro to the radiator and the case very well (like with epoxy). Otherwise, with the thermal changes that will happen, the velcro may come loose of the rad. or case and then it wont work
Good luck. Let us know how well it worked out.