You could certainly save and re-use the following search in Where in OF:

tasks where (allows next action of its context is false) and (start date is missing value or start date < now)

but at the moment Where in OF only displays its results in Project View, which I think you would find messy for this application. (Though the script does show a clean list of matches before offering to select targets in OF itself).

(I disabled the option to display in Context view because there are some marginal cases of task types which can not be seen in Context view. Perhaps I should re-consider this, and get the script to throw up a warning if it hits such cases).

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