As per our conversation, the only specific reference to the relationship between the Siyad Barre government and the 'Umar Mahamuud lineage of the Majerteen found among the sources currently available to the IRBDC is attached to this response. It consists of three pages from the 1984 book Somalia: Nation in Search of a State (156-158).
According to the attached document, the 'Umar Mahamuud was once a powerful primary lineage of which president Barre was said to be "particularly paranoid" (Laitin & Samatar 1984, 156). The same source adds that the SSDF rebel group is or was viewed "as a Majerteen party, in particular of the 'Umar Mahamuud" (Ibid, 157). Finally, the book describes the 'Umar Mahamuud as "the regime's second major opposition" and adds that it is segmented into at least three main branches (Reer Hirsi, Reer Khalaf and Reer Mahad) which are in turn divided into numerous "splinter groups" which may help explain a power struggle among the SSDF leadership (Ibid, 158). While discussing that power struggle, the attached source reports the imprisonment of Colonel Abdullaahi Yuusuf by Ethiopian authorities (both the SNM and the SSDF have reportedly operated inside Ethiopian territory). Other sources report the arrest of Colonel Yuusuf. However, other references to this event mentioning subclan rivalry, particularly involvement of 'Umar Mahamuud members, and further information on the requested subject could not be found among the sources currently available to the IRBDC.
Bibliography

Laitin, David and Said Samatar, 1984. Somalia: Nation in Search of a State. Boulder/London: Westview Press/Gower.

Attachments

Laitin, David and Said Samatar, 1984. Somalia: Nation in Search of a State. Boulder/London: Westview Press/Gower. pp. 156-158.