Is it possible to force a specific development branch of a package in Project.toml file?
No, the project file includes no such information. You will need to combine it with the manifest.
Using Binder, I also stumbled upon this. @fredrikekre is it complicated to add the support for branches here in compat?
I.e. having something like
[compat] Trebuchet = "master"
would already do the trick for me.
Luckily, binder also supports Manifest, so for now it is not crucial, however I rather prefer not to sync Manifests to githubrepositories and hence would be glad about Project.toml support