Is it safe to use a heating pad on an inflamed spleen?

Cold pack better: Heat often increases blood flow to an area, which might make the spleen tend to enlarge even more (and usually hurt more at minimum). Cold pack (wrap it to prevent skin injury) likely better relief w/o this risk. Be gentle until swelling down due to rupture risk & follow closely w/your dr.

Generally yes: Never on high heat! or while sleeping! one can get burned. If it is comfortable it is unlikely to be harmful anywhere. ( except diabetic, or neuropathic feet where real damage can occur since the feeling is diminished or absent).