I understand Wave doesn't offer Project based allocations or reporting so I've been looking around at a way of creating sub accounts or alternatively getting accounts to role up in a hierachy. Is this possible?
It isn't, for the time being. You could make unique accounts on your Chart of Accounts for specific projects and archive them once the project is over, but you won't be able to create an account hierarchy, nor does Wave currently offer sub-accounts as a feature.
I'd love to hear a bit more about your business. What kind of business do you operate? What kind of insight are you looking for with using project tags or sub-accounts? The more you can share about your needs, the better we'll be able to understand how impactful a feature like this one would be to your day-to-day, and to the work of other users as well.