Addition of new choices/items to a list in select widget during data collection

What is the general goal of the feature?
I would like to add an additional item or choice to a list in a select widget during data collection.
For any list to select from, the feature enables updating the list during data collection to make the options on the list more and more exhaustive as data gets collected.
What are some example use cases for this feature?
I am assessing brands in the market of a specific food. A list of known brands are port of the list, but new brands my be found during data collection.
If a respondent does not find their answer as a choice in a multiple choice question (list of brand names), I would like them to be able to add their option (new brand name) as an additional item to the list so that it is part of the choices for future data entries.

Another example where this could be useful would be identifying varieties of species of birds or plants in a specific area.

FYI this is effectively the XForm open selection, specifically:

... In this case, control select may have attribute selection="open". The form control should then allow free data entry, as described in 8.1.2 The input Element. The form control may permit multiple values to be entered through free entry.

which applies to both select (select_multiple) and select1 questions. Unfortunately, ODK Collect and Enketo do not currently support specifying selection="open" on selects. Instead, you can accomplish a similar result by adding an additional option - eg "Other" - and using this to trigger showing a following question to enter the custom value, as described here.

Which is to say, there is already a pretty well-established workaround for the absence of XForm open selection support in ODK [sic].

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