In contradistinction to universalists who may be pluralists or adherents to other religions, an evangelical universalist has to offer a method for understanding the "hell texts" of the Bible which both takes them seriously but is compatible with a dogmatic affirmation of the eventual salvation of all persons.
One approach that seems to me plausible is to understand the warnings of an eternal hell as specifically crafted with a particular rhetorical goal in mind, namely to motivate repentance and belief in Jesus Christ as Lord. In order to do this, the listener is put in a situation in which she must make a choice as to how she will develop her identity from this point forward, what sort of person she will be: either you pledge loyalty to Jesus Christ, or else you come under God's judgment. A person who constructs her identity in opposition to Christ, after having rejected him when presented with the choice, will not go on living indefinitely in autonomy and freedom in such an identity. There will come a moment when she will reap the consequences for her choice, for the person she has determined to be, and this will be in hell.
The universalist, then, will suggest that there are two ways of speaking of the experience, depending upon the perspective assumed. From the perspective of God, who sees something of value even underneath the surface of a falsely and poorly constructed self, it is a matter of purification and cleansing through fire. (Compare this to C.S. Lewis's suggestion that for those who escape hell, it will turn out to have been a purgatory of sorts, whereas it is hell for those who remain.)
On the other hand, it is destruction and unquenchable fire for the person undergoing hell, because she suffers punishment so long as she persists in her identity constructed in opposition to God. The language of the eternality of hell specifically is motivated by the phenomenology of the experience of judgment: it will be eternal because it will go on for as long as you do -- "you" here being understood as you-as-opposed-to-Christ. The "way out" of hell, so to speak, will have the phenomenology of death, insofar as it would mean the total abandonment of the conception of oneself that the damned person had constructed over the course of her life; it would be a sort of ego death or identity death.
The suggestion, then, is that the language of the eternality of hell is phenomenological, it is used insofar as it adequately describes the experience of hell that the damned person will go through. But precisely because it is phenomenological language, it need not describe a literal truth or reality right down to the T. There is the possibility of describing the reality in other terms which are not compatible with the phenomenological description taken literally.
In this way the universalist can see the value of using the language of eternal punishment, at least as addressed to unbelievers and the wayward and hardened hearts. For such a person the threat may be important. Importantly, however, that kind of language probably should not be used in the case of Christians, who have at least made the step forward in constructing their identity in favorable relation to Christ. Christians should not be threatened with hell but rather met with the grace and kindness of God who can lift them up.