With BBEdit as the active app and running the script from Butler, if I leave out the activate line, the choose from list dialog doesn't get focus regardless of whether I'm telling System Events or SystemUIServer to display it. That in turn means I need to re-activate the front app even if the user cancels, so in the bottom section of the script it's more convenient to leave the if block inside the tell block rather than vice versa.
Turns out I was being dumb about the boolean test — the test wasn't actually failing at all, I was just forgetting to coerce myChoice (a single-item list) to text before setting the clipboard. Duh.
To Chris's suggestion of having a default item in the choose from list dialog, I can type-select without it. Nice to know that's there, though — before I was just arrowing through the whole list.
So, no ready-made ideas for fully restoring the clipboard if it starts out as something other than plain text but coerceable to text? (Like I said before, it's not worth the effort of finding/developing a solution if there's not already a drop-in solution.)
So here's my latest version of the script:
tell application "System Events"
set frontApp to (file of first process whose frontmost is true) as text
try
set fullClipText to (the clipboard as text)
activate
set myChoice to (choose from list (paragraphs of fullClipText))
on error
set myChoice to false
end try
end tell
tell application frontApp
activate
if myChoice is not false then
set the clipboard to (myChoice as text)
tell application "System Events" to keystroke "v" using {command down}
delay 0.1
set the clipboard to fullClipText
end if
end tell
Michael