Show the softkeyboard with only alphanumeric keys?

Did you give a try to android:inputType="textVisiblePassword".

You can also do it programatically, without changing any other aspect of the EditText with:

editText.setRawInputType(InputType.TYPE_CLASS_TEXT | 
        InputType.TYPE_TEXT_VARIATION_VISIBLE_PASSWORD)

It will show something like this:

I think, the only way to achieve what you want it's to create a Custom SoftKeyboard with a custom KeyboardView class.
Here is a tutorial: Custom keyboard and see this SO question: Android app specific soft keyboard especially this answer which links to a open source project.