I lack perspective on this, and the relevant studies I can locate and read are often not specific about *how* signers are prompted to produce, e.g. Tyrone & Mauk (2010) doi.org/10.1016/j.wo..., who just say "asked to produce" here.

not trying to design a study, but rather trying to understand the (multi-modal) lay of the land for a survey article I am working on. insights appreciated!