If you enter OSR in the search tool at the top right of this site, you will see several discussions about it. The basic summary is that you just harvest it quickly, as you would have to do if you had traditional wax frames. No difference really. Here are some links to discussions, but there are more:
http://forum.honeyflow.com/t/flow-hive-in-the-uk-will-it-be-successful-with-rape-seed-as-the-primary-source/9058/2?u=dawn_sd
http://forum.honeyflow.com/t/uk-flow-keepers/423/97?u=dawn_sd
http://forum.honeyflow.com/t/jellybush-manuka-or-crystallised-honey-in-the-flow-hive/744/28?u=dawn_sd
Hope that helps. 