Reporting back on this. Over the winter I practised with flash positions and strength on my wife (who isn't bald!) using firstly B&W film and then slide film to make sure the exposures were good. Last week I visited some elderly relatives to shoot the portraits. I dealt with thinning hair by lowering the flash to eye level, I used a diffuser and positioned the flash about a meter away. I asked the subjects to slightly raise their chins. The results are good, no highlights burning into scalps and no wattles.