Of course, you are aware that this is an infinitive - "Words fail me."
Again, of course, you would be aware (just stating matter-of-factly, JIC other readers are not aware), an "infinitive" means infinite in time. Its assertion is unrestricted by time boundaries. Perhaps, indefinite with respect to time. An infinite assertion could be terminated past (had always occurred in the past until a certain point in history), terminated future (has started to occur infinitely after a point in history), or non-terminal (I am the alpha and omega).
Let's analyse some of the tenses.
infinite phrase; past tense; and completed past of a past (aka past perfect)
"Words fail me (all the time)."
"Words don't usually fail me, but words did fail me yesterday. Words failed me yesterday."
"Words should not fail me, but on the night of our election, words had failed me."
If I understand the question (or, if I did understand it), this is how I would phrase indirect reporting speech for the above cases:
1. Indirect reporting of infinite expressions.
1.1 Infinite reporting of an infinite expression. The reporter is using the infinitive to report an infinite expression.
"He says that words fail him. He always says that words fail him. He always says that words fail him all the time."
1.2 Past reporting of an infinite expression. The reporter reflects upon a past instance of an infinite expression.
"He said that words fail him. He said that words fail him all the time."
1.3 Past perfect reporting of an infinite expression. The reporter reflects upon a past instance of having reflected upon the occurrence of an infinite expression.
"He was drunk that night. He reflected on his past, of times when he had said that words do fail him all the time. Even prior to that night, he had (many times before) said that words fail him.
1.4 Past retrospective reporting of terminated past infinite expression.
Notice that we presume that the dead can no longer practice infinite expressions. Notice that the present subjunctive "would fail" is used here. The reason is that the past retrospective should not report an infinite expression. It speculates on an event that had not occurred, but could have occurred, which in turn would have speculated about a speech that could no longer have any infinite expressions. Unless this is a meet-up of voodoo reincarnation spiritualists, it would be more comfortable to the audience that the infinitive be converted to the (im)possibility of an infinitive. The intention is to respectfully project a spirituality-neutral mood.
"My dad should have been with us on that victory night. If he had been with us, he would have said that words would fail him."
1.5 Present subjunctive reporting of infinite expression. In comparison to the previous case, despite the fact that the non-voodooistic dead could no longer participate in infinite expressions, the speculation is on his being alive right now and that for such a possibility there would exist the possibility of non-terminal infinite expression emanating from him. The intention is to hypothetically presume his being alive and then place the non-terminal infinite expression into his mouth. So, I don't think the following would sound disrespectful.
"If he were alive, he would say that words fail him."
It also leaves room for us to perform retrospective speech, without confusing the two.
"If he were alive, he would say that words fail him. He had never drunk whiskey. If he were alive, and if he had had whiskey before, he would have said that whiskey would fail him."
1.6 Present subjunctive reporting non-terminal infinite expression.
OTOH, for a person who is still alive, no issues with termination of infinite.
He is unable join us today. If he were here, he would say that words fail him.
2. Reporting of past expressions.
2.1 Infinite reporting of past expression.
He always says that words failed him that night.
2.2 Past reporting of past present-expression. The confession was made immediately after his failure. The expression describes the past, but it was present time to the confession.
He said that words failed him.
2.3 Past perfect speculative reporting of past expression. The past of a past can only speculate on its future past, presuming the impossibility of time travel to its future. There are certain plans to drop the bombs. Which could be preempted by other possibilities.
In 1942, Oppenheimer had reflected on the future bombing of Hiroshima and Nagasaki. Amongst his daily notes of that year, he wrote that he had said that words would fail him when the moment should finally come."
2.4 Optative reporting of past expression.
He would like to say that words failed him.
3. Reporting of past perfect expressions.
3.1 Infinite reporting of past of past expression.
He says that words had failed him.
There are so many combinations. The above is a selection of them. I have missed out on the terminated future. I also did not illustrate a complete set of tenses. The above should help you dig into the patterns.