Age and sex are included automatically in the downloadable demographics file. But, typically one is expected to report other variables (e.g., race). Am I right in thinking that one could collect such data by adding a screening question to the study, but allowing all answers to the question? (E.g., screen for race, but allow all answers to the race question.) The aim would be to get participants’ answers to the screening questions without having to re-ask and without constraining the participant pool.
Yes, you are correct. However, your study pool will still be limited to people who answered the question. If someone chose not to identify their race, they will not be sent the study.
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Dear @Michael_Prinzing (& @Adam_Goodkind ),
what you guessed, and Adam confirmed, is correct. This is also what I do when I do not want to lengthen the study with questions that are already available in Prolific. 
Please consider that these pre-screeners are replied to by participants at the time of subscription to Prolific, unless the screeners are part of those subject to a routine check for which participants are asked to update their answers (there used to be a discussion on how to improve this mechanism ).
For ethnicity, there should be no concern about how frequently respondents renovate their replies (my guess is that it’s a “once and for all” reply given at the time of subscription). If you have concerns on the date when respondents answered particular screeners related, for instance, to work, beliefs, etc., Josh used to give us the intervals at which each screener must be updated. I guess @Jon —our new Community Manager— can do the same, if you provide him with a list of the ones you need.
Hope it helps!
-Veronica
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This is very helpful! Thank you!