Are there any aids (like a padded "belt") that can ease hip discomfort during aerobatics?
You can try out a padded harness which may help, a variety of places mention the stuff offered by Hooker Harnesses and may be what you are looking for. 



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If you currently only have a lap belt installed in the aircraft in question you may want to consider 5 point harnesses as they can help by moving some of the load to your shoulders.