For general purposes, I would say that the equivalent for this would be something like "You can't take it with you", or even "Eat, drink and be merry, for tomorrow we die". The page you cite goes on to explain the sense of the saying:

"nos sugiere que disfrutemos de esta vida, aquí y ahora, porque después de haber abandonado este mundo de nada sirven las riquezas ni los honores. [...] Nos aconseja que no esperemos nada de la posteridad después de muertos. Los elogios, la fama, la gloria son como la cebada que colocamos junto al rabo de un burro muerto. No sirven para nada."
http://www.inmsol.es/cursos-espanol-espana/burro-muerto-ceba...

So it's got an element of "agua pasada no muele molino", for which "closing the stable door" would be suitable.

But here, in this context, I think any of these would miss an important point, which is that the General's widow is saying that the nice things people say about him are no use now he's dead, but also, by implication, that her deceased husband is a metaphorical dead donkey. The latter could be a pointed remark or an unconsciously humorous one; either way, it is quite double-edged, and if we miss this something important is lost. It's also very down to earth and homespun in this solemn context.

So I think the above saying, quite close to the original, would be very suitable. This question is, is it a real saying? I've found it quoted as an "old Irish saying":
http://www.unexplained-mysteries.com/forum/index.php?showtop...
http://forum.wordreference.com/showthread.php?t=53711
If this is true, it's odd that it doesn't seem to be quoted anywhere else. But actually I think it would work even if someone actually made it up. It's pithy, and sounds like a traditional saying. It fits the context to a T.

an old Irish saying quoted here: http://forum.wordreference.com/showthread.php?t=53711

there seems to be some irony here: maybe the widow is comparing her late husband to a donkey through some subconscious urge without even realizing she's doing it? Why otherwise would she compare her dead husband to a donkey?

This may be more universal, beyond the British Isles...

¡suerte!

After Meat, Mustard

In Latin, “Post bellum, auxilium.” We have also, “After death, the doctor,” which is the German, “Wann der kranke ist todt, so kommt der arztnei” (when the patient's dead, comes the physic). To the same effect is “When the steed is stolen, lock the stable door.” Meaning, doing a thing, or offering service when it is too late, or when there is no longer need thereof.

Read more: After Meat, Mustard — Infoplease.com http://www.infoplease.com/dictionary/brewers/after-meat-must...

Just another one.