Dear Members of Indology,
Roland Steiner was so kind as to refer me to Wezlers article “Zu der Frage des Strebens nach äußerster Kürze– in den Śrautasūtras. Zeitschrift der Deutschen Morgenländischen Gesellschaft 151 (2001), p. 351-366, p. 360, note 45, which actually is the article I had in mind.
Wezler observes (on the basis of limited evidence) that colophons and other Sanskrit sources usually use the plural kārikā-s when referring to versified works, and he suspects that the usage of the singular became common only due to modern Indological catalogueing and the preparation of title lists.
With regard to the Sāṃkhyakārikā-s Wezler notes that the title provided in the work itself is Sāṃkhyasaptati, and that the commentaries, as far as Wezler could see (which coincides with my own observations) never refer to the work with the title Sāṃkhyakārikā.
With many thanks to all who responded on- and off-list.
Philipp