Hi,
First off I'll start by saying I haven't tried this so don't know how successful it will be...
The problem is the tk2dUIItemBoundsHelper code is part of the Editor and as such can't be called at runtime. However, the class is a static class so doesn't depend on an instance of it being created, so you could try copying the code into a non-Editor class (changing the name of the class obviously) and tweak the code where necessary need to get the same functionality.
Unikron may be around shortly with a better idea, but you can try that in the meantime :-)