Does anyone know when this practice started? Is it actually in the Typicon?
Truth be told, I've never seen EITHER σῶσον ἡμᾶς or πρέσβευε ὑπὲρ ἡμῶν inserted in the petitions in any official liturgical books. Some chanters and choirs just add it.
My personal guess is that it was introduced some time relatively recently as a liturgical echo of the Akathist Hymn/Small Paraklesis to the Theotokos. The construction "Most Holy Theotokos, save us" or "Saint X, intercede for us" only appears in Canon-like liturgical poems.
In the Small Paraklesis itself, it's always σῶσον ἡμᾶς, an imperative meaning "save us", i.e. come to our aid, like one would call out if stuck in a burning building. I can think of one Theotokion that uses πρεσβεύω in a participial construct specifically because the imperative (save us) is directed at Christ and the "interceding" is done by the Theotokos "...δέξαι τὴν τεκοῦσάν σε Θεοτόκον, πρεσβεύουσαν ὑπὲρ ἡμῶν, καὶ σῶσον, Σωτὴρ ἡμῶν..."