In short, at least in Europe no, because in Europe succession laws are usually not set by monarchs. If it was a monarchs powers it woudl also be in the regents power, but this isnt so. In other monarchies, where succession is maybe essentially ruled by the monarchs decree, the regent (having all the monarchs authority) coudl do so. If its not a restricted regency, of course...
I would consider that there would be some succession line already drafted out. The Regent can not name a successor to the crown on their own will, unless they've got the other likely candidates quaking in their boots. The Regent would usually only rule or represent the crown until there was a candidate of appropriate age.
Well my plan for my first timeline was to have Alfonso II d'Este, Duke of Modena and Ferrara die but he had a baby with his first wife Lucrezia de' Medici before his death. When he dies he puts Lucrezia as regent, but she see that she and her baby are slowly dying (form tuberculosis). So in order not to have Austria gain more land and supporters in Italy she makes her father heir to her son. But I think that what people are saying is this would not happen. I just saw your post V-J so it is around 1560 in Italy .