This is a great question. Here is the short explanation.
Project configurations will work across multi-project solutions, however the project configuration must reside in the root of the project (i.e. not inside a folder). Furthermore, the project configurations must be named identically across all projects. In the attached screenshot I am showing the project configurations DEV, PROD, and UAT all residing in the root of each project. I have highlighted the configurations folder so you know that is where not to place them.
Solution can have Dev, SIT, UAT specific configuration files (like Dev.os, SIT.os). This files will have environment specific URLs of each projects or each application. This value can be referenced in Global Container in String variable (like str_WebURL). String variable can be referred across all projects.
I have followed sasnt approach in v8.0.1053 and v8.0.1059 and I did manage to notice the Global variable being set based on the value in the Configuration item. However after a couple of runs this is now failing. I will be investigating this issue as "SubProject" configurations are very important in giving the selective ability to switch between environments.
Furthermore investigation has been carried out on this. When deploying the solution this seems to work fine, but not when debugging in Studio. So the issue seems to be isolated when using Studio and not Runtime (when Loading Local Project).