BIG-IP : iControl LocalLBDataGroupFile.set_local_path()

"...seen the corresponding data-group become unavailable to the iRule."

Really? Thats very surprising. Adding entries to a datagroup from the GUI should never take it offline.

"eg. attempt live-swap of data-group's data-file..."

Why would you need to do this? The F5 has a running config and a stored config. When you update a datagroup which points to a file you only need to update the file, then perform a config load to have that new files contents loaded into memory. It is also indexed at this time.