Brunswick also works for Ayling – the former British Airways CEO – in his capacity as chairman of Holidaybreak, and was used to announce his arrival at Sanctuary in May.
The arrival coincided with Merlin ending its ten-year relationship with the music group.
But Brunswick – which was unable to comment – is likely to secure the retained account despite not appearing on an original three-way shortlist.
Sanctuary manages acts including Joss Stone, but has had financial difficulties.
This month the group sold its MW Entertainment arm to singer Beyoncé's father, Mathew Knowles, for £2.7m – three years after buying it for £6.6m.