Figured out that since its not possible to change the text, its better to wire up a new button the same way as the older one inside the custom datasource's initWithModel method:
#pragma mark - NSObject methods
- (id) initWithModel : (id)model
{
if ((self = [super initWithModel:model]))
{
[IBAInputManager sharedIBAInputManager].inputNavigationToolbar.doneButton =
[[UIBarButtonItem alloc] initWithTitle:@"Dismiss"
style:UIBarButtonItemStyleDone
target:[IBAInputManager sharedIBAInputManager]
action:@selector(deactivateActiveInputRequestor)];
...
Let me know if this has repercussions that I cannot yet foresee.