Ossias can show staff labels like normal staves, the only difference being that staff labels on ossia staves are typically drawn within the system to the left of where the ossia starts, even if this is in the middle of a system, rather than behind the initial barline.
Ossia staff labels use the Ossia Staff Label font style, which you can edit in the Edit Font Styles dialog.
Ossia staff labels are positioned according to the distances set in . There are separate options for labels before single ossias and braced two-staff ossias.