AHM, Frank and Medibank all have current offers with 2+6 month waiting periods waived on extras when you purchase combined hospital and extras insurance.
Reading their T&Cs (e.g. AHM – https://www.ahm.com.au/offer/1-Month-Free-WW-Feb-5-2021) it seems like someone could sign up, claim extras, and then cancel or change to extras only cover.
Has anyone done this? Is there any reason why I couldn't?
There really isn't any reason you can't, up to your policy limits. After 6 months, the 'benefit' is gone anyway.
You just have to consider though that Hospital cover is what the ATO uses to determine whether you need to pay the Medicare Levy Surcharge if that applies to you by age and/or income.