As of now, the option to show the balance forward for a specific project in the invoice is currently not available. Don't worry though, I'll take note of this and pass it along to our product engineers. They're always looking for ideas to improve our product.
In the meantime, you'll want to check our QuickBooks Blog page to know about the latest news and updates in QuickBooks.
You might also want to visit our Help articles page to read some great articles. From there, you can learn some best practices in managing your QuickBooks Online (QBO) account.
If ever you need something to accomplish with, you can always post here anytime. I'd be here to assist you anytime.