Are there any Egyptian, Babylonian, Assyrian, or any foreign account of the demise of the Hittites?

Adding to @Timothy's answer: I concur that there are no detailed accounts of the destruction of the Hittites.

However, there appear to be at least two very, very short and incomplete accounts:

1. Ugarit was probably under Hittite suzerranity, but maybe not. In any case, it was not far from the core areas of the Hittites. A desperate letter written by king Ammurapi of Ugarit survives, in which

   • he refers to enemy ships that came and did many evil things in the country. He is very vague on this. He does not identify which enemy it was and he also does not detail what exactly they did.
   • he explains that his own army could not defend the country as they were away fighting for the Hittites. (Literally, in Hittite lands. I would take this to mean that they fought for the Hittites, not against them.)
   • Most of the text is quoted on the wikipedia page on Ammurapi in English, only the greeting and the ending are missing. The greeting reads a bit effusive but is otherwise not remarkable; in the ending, Ammurapi requests to be kept up to date about enemy ship movements.
   • The letter is addressed to the king of Alashiya (Cyprus). Wikipedia says, it was sent in response to a plea for help (in which case the letter declines to help and explains why this is presently not possible). However, I cannot find any reference to a previous plea for help in the text of the letter. (Edit: Contrary to what the wikipedia page on Ammurapi says, it seems to be the other way around: Not Alashiya requested help from Ugarit, but Ugarit from Alashiya. The king of Alashiya replied with letter #23 on pages 85-86 of Nougayrol et al.'s book quoted below, suggesting that Ammurapi should use his own soldiers to defend his city. The letter in question here seems to be a second desperate plea for help, which also explains the effusive greeting.)
   • The full text of the letter can be seen in original (transliterated from Ugaritic cuneiform) and in French translation on pages 87-89 of Nougayrol et al.s (1968) Ugaritica. V: nouveaux textes accadiens, hourrites et ugaritiques des archives et bibliothèques privées d'Ugarit (pages 103-15 of the pdf).

Au roi d'Alašia, man père, dis: ainsi parle le roi d'Ugarit, ton fils: Aux pieds de man père je m'effondre. A man pere, salut! A tes maisons, tes epouses, tes troupes, à tout ce qui est au roi de l'Alašia, man père, grandement, grandement, salut!

Man père, voici que des bateaux de l'ennemi sont venus: des villes miennes par Ie feu il a brûlé et des chases bien déplaisantes dans le pays ils ant fait. Man pere ne sait pas que toutes mes troupes en pays hittite stationnent, et que taus mes bateaux en pays lycien stationnent. Jusqu'a présent ils ne me sont pas parvenus en retour, et le pays est ainsi abandonné à lui-même, Que mon père sache cette chose-là! Or, c'est 7 bateaux de l'ennemi qui me sont venus sus, et ils nous ont fait de bien mauvaises choses.

Maintenant: s'il y a d'autres bateaux de l'ennemi, informe-m'en de quelque manière, et que je le sache!

I refrain from quoting the Ugaritic original but it can be found in the same book.

2. Pritchard's (1969) Ancient Near Eastern Texts Relating to the Old Testament with Supplement page 262 quotes an inscription by Ramesses III in Medinet Habu:

The foreign countries made a conspiracy in their islands. All at once the lands were removed and scattered to the fray. No land could stand before their arms, from Hatti, Kode, Carchemish, Arzawa, Alashiya on, being cut off at one time. (...)

This is basically the introduction of another glorious account of Ramesses III's victories against the invaders. I would not necessarily take it at face value (the Egyptians occasionally took a rather liberal approach to embellishing the truth), but it does suggest strongly that the Hittite state did not exist eny longer.

Some additional aspects:

• Scribes at the time were very rarely concerned with writing about political/economic/social developments in other countries. The overwhelming majority of the surviving documents are either of practical nature (letters, inventories, ...) or they brag about the accomplishments of the one who commissioned them.
• Especially in the case of the Hittites, it is extremely difficult to date written documents, as they did not use a dating system. (See Beckmann (2007) From Hattuša to Charachemish: The Latest on Hittite History)
• The Late Bronze Age collapse was a major break in all states of the middle East from Mycenae (Greece) to Elam (Iran). I am unsure about other regions. The wikipedia article features a map that emphasizes how little actually survived. People, including most rulers, had more urgent things to do than to commission artworks to celebrate their accomplishments.
• As mentioned above, there is evidence for foreign invasions both in Egypt and on the margins of the Hittite Empire, namely in Ugarit.
• With regard to the core of the empire, the capital Hattusa, the Encyclopedia of Ancient History (Gary Beckman) claims:

Although the uppermost Hittite levels of he site do reveal extensive burning, the damage does not seem to be attributable to a single final conflagration. Rather, its excavators now believe that Hattusa was gradually abandoned over the final years of its existence.