Progress have supplied the answer
Progress have given me the answer:
rfutil <db> -C aimge extract -a <ai_extent> -o <filename>
Where <ai_extent> is an existing AI extent file that is (status) "FULL".
rfutil will extract from the AI file only the active AI information and copy it to the new file.
The reason why this is important (for some people, anyway) is that using Variable AI extents imposes a significant perormance problem ... but if you use Fixed extents you waste lots of file space. It is an extra problem for us since we then send the AI file over a not-so-fast comms line.
This new feature solves the problem by copying a Fized extent only to the limit of the actual (used) data in the AI file.