According to the Catholic Church, are a married couple permitted to engage in oral sex when sexual problems such as impotence make vaginal sex impossible?
Canon Law and the Catechism both have something to say on impotence.
If the impotence developed prior to the marriage, the marriage is invalid (Canon 1084 §1).
The Catechism insists that marriage is open to fertility.
By its very nature the institution of marriage and married love is ordered to the procreation and education of the offspring and it is in them that it finds its crowning glory."160
Children are the supreme gift of marriage and contribute greatly to the good of the parents themselves. God himself said: "It is not good that man should be alone," and "from the beginning (he) made them male and female"; wishing to associate them in a special way in his own creative work, God blessed man and woman with the words: "Be fruitful and multiply." Hence, true married love and the whole structure of family life which results from it, without diminishment of the other ends of marriage, are directed to disposing the spouses to cooperate valiantly with the love of the Creator and Savior, who through them will increase and enrich his family from day to day.161
The fruitfulness of conjugal love extends to the fruits of the moral, spiritual, and supernatural life that parents hand on to their children by education. Parents are the principal and first educators of their children.162 In this sense the fundamental task of marriage and family is to be at the service of life.163
Spouses to whom God has not granted children can nevertheless have a conjugal life full of meaning, in both human and Christian terms. Their marriage can radiate a fruitfulness of charity, of hospitality, and of sacrifice.
All of this points to sex being used for the procreation of children. The Catechism is also a little more explicit on sex for its own sake. I'm not entirely sure that the concept of oral sex had reached the hallowed halls of the Vatican when Pope S John Paul II promulgated the Catechism in 1990, but since it's effectively a form of masturbation, particularly in the wide definition the Catechism uses, this appears to be the nearest text:
By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure. "Both the Magisterium of the Church, in the course of a constant tradition, and the moral sense of the faithful have been in no doubt and have firmly maintained that masturbation is an intrinsically and gravely disordered action."137 ... Here sexual pleasure is sought outside of "the sexual relationship which is demanded by the moral order and in which the total meaning of mutual self-giving and human procreation in the context of true love is achieved."138
To form an equitable judgment about the subjects' moral responsibility and to guide pastoral action, one must take into account the affective immaturity, force of acquired habit, conditions of anxiety, or other psychological or social factors that lessen or even extenuate moral culpability.
Consequently the answer is No: they are not permitted to engage in oral sex. There may be circumstances where culpability might be lessened, but that does not mean the practice is allowed.
The Church recognises through the Catechism that childlessness is a great burden.
Couples who discover that they are sterile suffer greatly. "What will you give me," asks Abraham of God, "for I continue childless?"163a and Rachel cries to her husband Jacob, "Give me children, or I shall die!"164
Research aimed at reducing human sterility is to be encouraged, on condition that it is placed "at the service of the human person, of his inalienable rights, and his true and integral good according to the design and will of God."165
The Catechism goes on to consider IVF, which has been the subject of other questions here. However, if impotence is merely a matter of erectile dysfunction (as this question implies) then medical interventions to counter that and allow normal sex are not forbidden.
137 Persona humana 9 (Congregation for the Doctrine of the Faith)
138 Persona humana 9 (CDF)
160 Gaudium et Spes 48 §1; 50
161 GS 50 §1; cf Gen 2:18, Mt 19:4, Gen 1:28
162 Cf Gravissimum educationis 3
163 Cf Familiaris consortio 28
163a Gen 15:2
164 Gen 30:1
165 Donum vitae intro 2 (CDF)