Question

Is there any way to load an installed activity that isn’t explicitly defined in my application manifest?

Answer 1

Sure. You could start the "gallery" to pick a photo like this:

myIntent = new Intent(Intent.ACTION_PICK, android.provider.MediaStore.Images.Media.EXTERNAL_CONTENT_URI);  
startActivityForResult(myIntent, IMAGE_UPLOAD);

from the manual, we find you can have 2 types of intents to call:

Explicit Intents
have specified a
component (via
setComponent(ComponentName) or
setClass(Context, Class)), which
provides the exact class to be run.
Often these will not include any other
information, simply being a way for an
application to launch various internal
activities it has as the user
interacts with the application.

Implicit Intents have not specified a component; instead, they
must include enough information for
the system to determine which of the
available components is best to run
for that intent.

You are looking for implicit intents I guess. This is explained a bit lower:

The intent resolution mechanism
basically revolves around matching an
Intent against all of the
descriptions in the
installed application packages. (Plus,
in the case of broadcasts, any
BroadcastReceiver objects explicitly
registered with
registerReceiver(BroadcastReceiver,
IntentFilter).) More details on this
can be found in the documentation on
the IntentFilter class

(emph. mine).
So you need either to know something to call it explicitly (like classname etc), or call the intent, and then you’d have to know the intent-filter.

I don’t think there is a method to call something knowing only the package name: mind you, you can have an app with several packages, where not all have activities, let alone activities that can be run.