Using tank fuel as a heat reservoir can be dumb. You end up with ridiculous min fuel states. No problem using fuel on its way to burn, except, it has its limits. As far as avoiding heat signature, well, there is a whacking great big one that should be dwarfing the systems heat load!
Overall, don't blindly think of the fuel load as an easy heat sink, it can easily become a very heavy deadweight!
OAP