I’d avoid that…they’ll be a melted mess.
based on the above, I’d suggest ditching your plan of starting the race w/o your hydration pack. Just take it from the beginning so you have plenty of fluid from the get-go. throwing a bottle in your jersey is a poor substitute…and will bounce all over the place as you hit the flint rock.
You can chase calories if you fall behind and get back to where you need to be…but if you start chasing hydration early in the day at Unbound, you are just gonna suffer (and possibly DNF).