I wonder if “si tant est qu’il y en ait une” works as well

No! You were totally right in noting this point to the uncertainess of its existence. The meaning is totally different. Your

Voilà une piètre excuse, s’il en est.

is the best translation for me.

First, I agree that in English it is "hyperbole, singling out this particular excuse as the most flimsy one imaginable among all others," so the simplest way to express this in French might be to just spit it out bluntly (hyperbole or not) with:

Voilà/Ça c’est/Ça doit être/ la plus piètre excuse que j’ai jamais entendue.

I think you could also express this in French without the hyperbole by simply using some emphasis:

Ça c’est une piètre excuse! …

… to which you could perhaps add "ou je ne m’y connais pas" (from languefrancaise.net) to try to better match the English contruction:

Ça c’est/Voilà/ une piètre excuse ou je ne m’y connais pas.

Finally I think you could even get some ideas for this question from the comments and answer to your question about how to say-"if it isn’t x then what is it?"

To retain the hyperbole idea, you could use:

C’est sans (aucune / l’ombre d’une) comparaison, la plus piètre excuse qu’il soit.

or “que je n’ai jamais entendue” in a more casual register.