You can usually get away with it, unless the thermal tape is exceptionally sticky (might be hard to take off later on). But a lot of thermal tape is very thin, and many waterblocks assume a certain amount of clearance between it and the components on the PCB, to ensure to they get maximum block-core pressure. So you might have to double or triple layer the tape to ensure that everything else gets cooled too. It wouldn't be my first choice, but if you don't have any other options, you should be alright.
__________________ i7 2600K | ASUS Maximus IV GENE-Z | GTX Titan | Corsair DDR3-2133 |