On the reference selector of your project_newedit page you can use xpath on the selectable objects: https://docs.mendix.com/refguide7/reference-set-selector#selectable-objects-properties
In order for the xpath to work, instead of using the default owner association i suggested creating a new association, something like: Project_Account_Owner, and set this association on the creation of the object.
Then you can use the following xpath:
[YourDomainModel.Company_Account/YourDomainModel.Company/YourDomainModel.Company_Account/Administration.Account/YourDomainModel.Project_Account_Owner = '[%CurrentObject%]']
Mind you, because you are using xpath on the reference selector the project should be committed in the database in order for it to work.