I only know of one workaround, and it involves the creation of a Floating event either on your Macbook or any other iCal compatible thing that allows it. Create an event titled "FLOATING" and set it a date/time in the past. Wait for it to sync to your phone.
Now, on iOS, if you need to create a floating timezone event, start by typing "FLO..." into the title and iOS Calendar should find the one you created and allow you to select the pop-up details. This creates a duplicate. Now, careful not to touch the timezone info, edit the rest to your liking.
Quite why Apply refuses to allow Floating as a valid timezone on iOS is beyond me. It's the same stupid simplification philosophy they use that means one can't create an event on iOS that has different start and end timezones. Makes it useless for flights. One would assume at least one person who works at Apple has taken a flight ...