Oh, damn. Auto-mounting is controlled by acs-kernel/sql/*/acs-install.sql rather than the .info file.
And this of course is why the package instantiation callback is not invoked. There's no way to call a Tcl proc from SQL.
If you're talking about controlling auto-mounting as well as auto-installation via the .info file from the APM rather than hardwired SQL files, then by all means yes, this should be done.
And it should be done by following the full mounting drill including calling the post instantiation callback proc for the package being auto mounted.
(actually "auto instantiated" though for a singleton package they're the same thing since you can't mount such an instance at multiple points, or at least shouldn't be able to)
