The UITextInputAssistantItem contains 2 arrays, leadingBarButtonGroups and trailingBarButtonGroups. You can add instances of another new class, UIBarButtonItemGroup, to these arrays to add or replace items on the short cuts bar. UIBarButtonItemGroup has 2 major properties, barButtonItems and representativeItem, that you can in turn fill out to specify what is on the bar. Finally to hide the bar (assuming that predictive text is also off) you can set the leading & trailing groups to an empty array.
Example:
NSArray barButtonItems = @[ /* Create UIBarButtonItems and place them here */ ];
UIBarButtonItem representativeItem = // This is optional, if present when there isn't enough room on the bar, the group will be replaced with this item
UIBarButtonItemGroup *group = [[UIBarButtonItemGroup alloc] initWithBarButtonItems:barButtonItems representativeItem:representativeItem]
textView.inputAssistantItem.leadingBarButtonGroups = @[ group ]; // replace the items on the bar. Alternatively you can append this group and get the default leading items with this extra group
Hopefully that gets you started.