OK, I've made this an example sentence: https://tatoeba.org/eng/sentences/show/7316608
I could go either way, but I suppose this is best off there.

i agree.  if it's not in a dictionary, the right place for it is probably the example sentences.

I'm just generally opposed to adding long, non-idiomatic expressions that aren't in the refs, regardless of how common they are.

やらずに後悔するより	6533
やらないで後悔するより	4590
やらぬ後悔するより	No matches
やらずに後悔するよりも	No matches
やらないで後悔するよりも	No matches

I don't think "there are many variations" 
is a good argument for not including the 
most common variation. This saying is a 
whole lot more common than most of the 
proverbs/sayings that appear in 
daijs/daijr. I don't think how well it 
rolls (or doesn't roll) off the tongue is 
very relevant either, nor how recent it 
is.

I'm not sure this is worth including. It's a full, modern sentence, and not found in any refs. There are a lot of variants with やらないで、やらぬ、よりも, etc.
I feel a proverb needs to roll off the tongue more easily.