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