There are several issues here:

1) **Any mucus is a specialized tissue for absorption.**

Mucus indeed is not so good for passive diffusion, that makes it absolutely perfect tool for active absorption of certain substances, almost independently from the mucus type. To provide some examples: many drugs like cocain are inhaled and absorbed in nasal cavities, whereas rectum is also known and favorite delivery way for medicines. 

Generally the absorption force of the mucus is dependent upon how good it is vasculated, for after going through the basal membrane the substance directly enter the blood flow and quickly travel away, thus the concentration gradient on both sides of the basal membrane -- the main barrier in mucus -- remains relatively high.

See [this paper][1] for an experimental model for this statement.

2) **Mouth mucus can absorb many substances.**

There is a special term called "oral absorption" to describe the rapid drug absorption into blood flow from the mouth cavity. The mucus is not specialized here, but small molecules are able to permeate here through all barriers. 

3) **Advantages of oral absorption.**

There are some:

* *Mucus layer of mouth cavity is very highly vascularized.* The whole mouth can be seen as a bundle of skeletal muscles and every muscle requires a lot of energy and oxygen, therefore they have one of the highest vascularity rate, having much less distance between single capillaries.

* *Blood flow is higher in the mouth cavity walls than in other inner organs*. This is mostly because muscle contractions (during chewing) lead to increased propulsion of blood through capillaries and small vessels here.

* *Any substance that enters blood flow here bypasses the hepatic-portal system*. This means that the substance does not have to wait until it is filtered by our liver, it is immediately distributed through the whole body.

* *There are special reflexes from oral mucus to inner organs.* Even not so important in case of aspirin, this is very important for some placebo drugs like methyl valerate (known as "Validol" in many countries) used a lot for treating *angina pectoris* whose only action is to activate the cold receptors in mouth and thereby leading to reflectory dilating of cardiac vessels.

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This is why many drugs, like for example loperamid, are administered only as sub-lingual tablets. And this also explains why in emergency medicine many remedies are injected directly into the tongue.

  [1]: http://www.mendeley.com/research/limiting-role-mucus-drug-absorption-drug-permeation-mucus-solution-3/