I mean... there is no 'one thing' any Catholic has done. There have been faithful Catholic Lords/Monarchs, and there have been not-so-faithful ones. Making blanket statements like 'actual fasting was pretty much a poor people thing' is not really a good description. Fasting is really something only the rich can do anyway, since if you're already poor... you're not missing much. There have been very faithful Catholic noble families (the Habsburgs come to mind -- many are still quite faithful today!) and they certainly did the normal fasting.

That being said, I've never heard of replacing the fasting days with a 'repentance fee'. I'm not going to say it's never happened because there have been billions of Catholics, and probably millions of priests so maybe someone did that somewhere. However, under normal Catholic doctrine it's impossible to pay to repent. God's mercy is freely given without money.