In the meantime I have experimented with scripting DevonThink and it turns out that it is indeed easy to automate the process with an applescript like this one:
-- get references to the relevant groups
set theSmartGroup to get record at "/Label High"
set theDestination to get record at "/Sync DT with GR/High"
-- get references to the content of the Smart Group
set theContent to get children of theSmartGroup
-- replicate the items in the Smart Group to the priority group
repeat with thisItem in theContent
if not (indexed of thisItem) then replicate record thisItem to theDestination
end repeat
-- move the relevant files to the external folder
deconsolidate record theDestination