Well with a little AppleScript help from a forum user I cobbled together this script:
tell application “OmniFocus”
tell front document
set selectedTasks to every flattened task whose (flagged is true ) and (completed is false )
set oTag to ( first flattened tag whose name is “Next”)
repeat with i from 1 to count of selectedTasks
add oTag to ( tags of item i of selectedTasks)
It makes sure that every flagged item is also tagged with “Next,” my forecast tag.
Since I use flags to denote “Must Do’s,” it stands to reason that flagged items are a subset of “Next” items anyway, so if I set this script up to run periodically, then I think I should have achieved exactly what I wanted.
That said, I can barely Applescript my way out of an open box, so if anyone sees any issues with this please tell me!