My sister did the PHM this past February at 7.5 months pregnant. That being said she had more than a year of training under her belt before getting pregnant and just continued that conditioning and training. She also did run/walk intervals. What worked for her was a 30 second run/1 minute walk and that kept a 16 min/mile pace mostly.
I'm sure Disney would not encourage people to run these races while pregnant, but their waiver pretty much means you are at your own risk and they will accept no responsibility.
I think if you were training smart and doing run/walk intervals or walking the whole thing you could probably still do the race. But you need to listen to your body and decide when you get pregnant, taking into account how far along you'll be etc.
I know you can defer a half marathon registration to a later race, not sure about the 10k, but that might also be an option.