I just asked a related question, and in German, it is not entirely clear what two consecutive relative pronouns "was" each qualify.

{German}: Es gibt nichts, was er sagen könnte, was die Tatsache ändern oder auch nur rechtfertigen würde.

In English, on the other hand, two relative pronouns do not seem to give rise to ambiguity, especially since the first "that" can be omitted.

{English}: There's nothing (that) he could say that would change or justify the fact.

Then I wondered how I would express the same idea in French, and the following construction came to mind first. I notice I kind of finessed the issue of a relative pronoun pile-up by not using the second one.

{French}: Rien de ce qu’il dirait ne pourrait changer ou justifier ce qui a été fait.

But in spontaneous speech, you cannot always start a sentence ideally and yet might try to construct the rest of the sentence somehow with what you have started.

So what if you start off with "Il n'y a rien ..." instead and need to incorporate two relative pronouns somehow, just as is the case with the German and English sentences?

This might prove to be a more complex structure than the one I suggested above, but how would you go about it without causing ambiguity?

Il n'y a rien ... {with two relative pronouns}