Science Time: How is alcohol created in the first place? The easiest, and more natural way is through fermentation.
"As sugars are broken down by microorganisms for metabolisation (a process where they are converted into usable energy), various byproducts are produced. These "waste" compounds can be either gasses (like carbon dioxide), acids (such as lactic) or alcohols (in the form of ethanol). This is known as fermentation and a key process in the making of antiseptics, preserving spirits and a host of beloved adult beverages."
Now, are there any microbes can metabolise sugars efficiently without being choosy about the conditions in which they do it?
Yeast are small eukaryotes that are found almost everywhere on earth, reproduce at the drop of a hat and are excellent at manufacturing large amounts of alcohol in a very short space of time. Many of these organisms are classed as being anaerobic, meaning they does not require oxygen to live (to metabolise), and so can thrive in environments other bacterium fail to exist in at all. 344
To get the energy they require in these oxygen-devoid areas, yeasts convert sugars into energy through fermentation, producing ethanol as a waste product.
And finally: how does any of this have to do with the rats themselves?
What if the rat's digestive biology (stomach bacteria etc.) included yeasts as a staple organism? The stomach is a warm, dark, hostile and oxygen-lax environment perfect for yeast to cultivate in private, gorging themselves on any sugary materials that come their way.
By giving these rats a second stomach (a cheap goon bag if you will) lined thick with mucus, where in sugar rich foods can be sent and broken down by inhabiting yeasts, a substantial quantity of ethanol will be produced. This alcohol can then be stored safely in this stomach in much the same way HCl is in ours until dissipated, either very quickly through the rear end, or when it is really needed Ie. When the rat decided its time to be a drunken hero!
Through some circulatory/excretory process, when the rat is caught and about to be devoured for the greater good of it's colony, it will release the contents of it's second stomach either into it's bloodstream and main tissue mass (I mean, the creatures already going to die right?) or regurgitate it over itself, so whence eaten, delivers far more than the legal limit of booze into the predator, leaving it chill and mellow to be captured gnawed or whatever, by the rest of the party.
NOTE: The yeast's process of metabolisation also creates carbon dioxide (that makes baker's bread rise), so the rats are going to need to expel much gas during the fermentation process; so the booze-rats will not only be "drunk" in sense, but also be burping and farting a heck of a lot too! How weird would that be: a colony of groggy, flatulent rodents...
ADDITIONAL Q & A: A little off question, but what could attribute to the rats sacrificial tendencies? Is it purely selfless - morally justifiable "cause before self" - or is there some other biological incentive that fuels these creatures actions? The latter seems more plausible (and more widely discussed through the comments - thanks for the feedback guys).
The allure of the ultimate bender is too tempting for most to resist. Seeing as the rats do have their own lifetime supply of booze, it would be safe to assume that they have some degree of access to the alcoholic contents of their stomach (either by choice or because minute quantities of ethanol may be excreted naturally from it over time) and may therefore regularly indulge themselves on such, the rat gradually building a tolerance to ethanol as they grow older, just as some humans do by continually drinking.
Perhaps then, their heroic actions are not so selfless after all, instead simply a means of satisfying their ever growing thirst for ethanol through the biggest hit of booze they will ever have (the entire alcoholic volume of their stomachs being released into their bodies); all the rats would need for this to be achieved are ample dormant alcohol receptors (large untapped dopamine and serotonin stores) in their brains, that can only activated/accessed once the sacrificial pledge is made (hence, once evoked by a huge adrenalin or insulin spike accompanying the release of alcohol). It would not only utterly intoxicate the rodents with feelings of euphoria and unfounded courage, but also act as a pain inhibitor too.
This motive for sacrifice would work to the colonies advantage, for only the rats with a greater tolerance to ethanol - a desperation/drive for more - will offer themselves to a predator; these rats logically will be the oldest drunkards of the colony and so are at greater risk of dying naturally anyway (might as well go out with a bang...). 344