Well, yeh, that's differant... The UBB has a base path, the wrapper uses that, so if you just use a full path to the file you completely eliminate that as a possible problem...
By calling that page outside of the wrapper, you're going past the UBB and its base path...
I did find a couple of things on google for using recaptcha in smarty, basically putting your php code and making a variable for smarty then calling that variable in the smarty wrapped code...
clicky