Swallowing specialist here. I deal daily with people who can't take in food and fluids the normal way and must have nutrition/hydration through IV or feeding tubes.

In this article, the authors study Western European descriptions of coma from the past 300 years, including treatment attempted. In the descriptions they found various treatments were attempted, but a quick read through their article doesn't mention any attempt to provide alternative nutrition/hydration.

And frankly, I don't know how they would. I deal with patients who have varying levels of consciousness on a regular basis, and I can tell you that people in comas generally lose their swallow response.

The only ways of providing nutrition/hydration for someone who can't swallow are 1) feeding tube, either surgically placed or fed through the nose and down the esophagus, or 2) IV.

I have had patients for whom neither the feeding tube nor the IV was an option for whatever reason, and there was essentially nothing we could do. Which makes me highly doubt that the ancients had some other way.

Today the primary causes of death in comatose people are things like pneumonia. Considering dehydration would likely strike earlier, I'd bet the answer to your question would be yes, if they didn't get killed by bloodletting sooner.

EDIT: Interestingly enough, I found a study published in 2005 which looked at the effect of bloodletting on consciousness in patients with apoplexy... and reported that it actually helped!

EDIT 2: A lot of people have asked about the possibility of rectal feeding. Honestly, I'd never heard of it before. It's not generally used in modern medicine, presumably because nasogastric tubes and IVs are so much superior. This article claims that some ancients used rectal feeding, but does not mention whether it was used for unconscious people. If it was tried, I have no idea how effective it would be. Questions about rectal feeding might be better directed to /r/askscience

EDIT 3: Others have asked why the ancients might not have used a feeding tube similar to modern nasogastric tubes. Besides the fact that I am aware of no evidence that they tried such a thing, it would be very difficult.

First of all, you would need a flexible but firm tube that wouldn't collapse even when being squished by the upper and lower esophageal sphincters. What kind of material could they use for that?

Second of all, you have to understand that the tube to your stomach (esophagus) is closed by default, and the tube to your lungs is wide open for breathing. Inserting a feeding tube is difficult because the tube naturally tends to go towards your open airway. If you don't have modern x-rays and other visualization techniques to help guide you, you must have someone who is capable of swallowing the tip of the tube, and even then it very often goes into the wrong place.

The fact that your esophagus is closed by default is also why force-feeding someone in a coma would send the food straight down to their lungs, if they are unable to swallow.

Here's a very decent answer, taken from a similar thread. :) http://www.reddit.com/r/AskHistorians/comments/1qe1er/how_were_comatose_people_treated_in_the_past/cdbw823

To echo some of /u/420836764677's comments, the Roman's demonstrated that comas were somewhat synonymous with death.

According to both Tacitus and Seutonius, Tiberius collapsed and fell into a coma in 37 AD. Roman Senators and other officials began the effort to congratulate Caligula on his ascension to power, however Tiberius did get up from his coma. Tiberius was subsequently killed by Caligula's chambermaid (who smothered him to death), and did not die as a -direct- result of the coma, but rather a very obvious indirect reason.