You wouldn't need it on all the time, just warm enough to keep the cistern surface above the dew point. Tepid should be adequate.
A factor that the OP may not have anticipated (if he's serious) is that tepid water is an ideal breeding ground for all manner of bacteria, so he should find some interesting life-forms in the cistern after a year or so.
The common one is legionella; all you then need is something to create an atomized spray; flushing does that nicely, there is a mist of droplets dispersed over the adjacent surfaces after flushing. A visit from granny or grandad may end unhappily.
its not 100%, but its looking no worse after a year of abuse than it looked when I bought the house. Slightly raised around the bath/shower but otherwise OK.
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