Gareth, you know better than anybody that there is a huge difference in API between 2.3 and 2.4. And by "Flex" Deirdre meant flexsections, not flexpage. Flexpage is already available for 2.3. Flexsections will never work on 2.3 simply because the API did not allow that.
Basically a lot in the new course formats API in 2.4 was inspired by Flexpage. It is very comprehensive set of 5 different plugins (framework, activity, format, block and theme). Also I doubt Moodlerooms will release the 2.4 version of it simply because now the same can be achieved much easier.