Hi LXNDR
In fact, cittekaggatā is not as rare as some would believe. If we resolve the compound into its members and do a proximity search for the members in the suttas, we get -
cittaṃ ekaggaṃ (the mind [was] unified) in MN 4, MN 19
cittassa ekaggatā (unification of the mind) in MN 44, MN 117, MN 125, SN 45.28 (mirroring MN 117), SN 48.9 – 11, SN 48.50
ekaggacittā in SN 47.4 and AN 2.42 in the context of satipaṭṭhāna,
In fact, its occurrence as cittaṃ ekaggaṃ (the mind was unified) in MN 4 and MN 19 within the context of satipaṭṭhāna just before the 1st Jhana would suggest that it is not a quality that shows up only in the Jhanas.
Made a quick check of MN 4’s parallel in EA 31.1, and it also pops up there as 一心 as the precursor to the First Dhyana.
From the looks of it, the ekaggatā with reference to citta is not exclusive to or monopolised by jhana. It is also a quality shared by well-established mindfulness.