Prior to the revision of the Breviary instituted by Pope St. Pius X, the Octave of St. Peter Martyr would be celebrated today. This is one of the many octaves of the Dominican saints that were casualties of the revision of St. Pius X. Not all of the Dominican saints had octaves after their feast day, but the major ones like St. Peter Martyr did. I think this is something that could have been retained, particularly as it was merely a commemoration at Lauds and Vespers. For the life of me, I do not see the harm in adding a commemoration, particularly of a saint that exemplifies so much of what the Order stands for.
In the 1909 Breviarium S.O.P., the feast had the rank of "simplex", which would be considered a commemoration according to the rubrics of the 1962 Office. At Vespers and Lauds, an antiphon, versicle/response and collect pray are given in the Proper of the Saints. They are the same as those from the saints feast day on April 29.
Also, if you get a chance, check out this wonderful article that was recently posted about St. Peter, and how his feast is celebrated in Milan, over at New Liturgical Movement.
From the 1909 Breviarium juxta ritum sacri ordinis praedicatorum:
Commemoration at 1st Vespers:
Magnif. ant: O Peter, renowned martyr, glory of the Preachers, endowed with virginity, eloquence, miracles and grace, when our earthly life has been completed and the loving mercy to which we are accustomed has been granted to us, bring us to the heavens, alleluia.
℣. Blessed Peter pray for us, alleluia. ℟. That we may be made worthy of the promises of Christ, alleluia
Prayer: Grant, we beseech you, almighty God, that we may earnestly and with fitting devotedness follow in the faith of the blessed Peter, your martyr, who for the spread of that same faith merited to obtain the palm of martyrdom. Through our Lord...
Commemoration at Lauds:
Bened. ant.: Possessing the highest purity and distinguished by grace of doctrine, renowned by the victory of martyrdom, Peter is resplendent in the glory of a threefold crown, alleluia.
℣. A gold crown upon his head, alleluia. ℟. Inscribed like a signet with "Holiness," alleluia.
Prayer: Grant, we beseech you, almighty God, that we may earnestly and with fitting devotedness follow in the faith of the blessed Peter, your martyr, who for the spread of that same faith merited to obtain the palm of martyrdom. Through our Lord...