There are three "masking" options. The default is unmasked, the second draws * instead of each letter your type 'mask-all', and the third replaces the characters with *'s after you hit enter 'mask-submitted'. Perhaps that will give you some of the desired behavior? Another option is adding a time delayed callback to clear the text.
This is from my working application:
CODE
that.controls.password_text_entry = new KONtx.control.TextEntryButton({
id: 'pw-entry-button',
label: $_('password'),
secureMask: true,
secureMaskType: 'mask-all',
styles: {
vOffset: toff
}
}).appendTo(this);
Hope that helps!