This bug came to light a while ago and was filed somewhere but presumably it's never been fixed. That module uses a python library called formencode and for some reason the regex code in that library fails to account for explicit port numbers in the URL.
I think your best options at this point are
b) Copy the files that comprise the IFrameInclude module, tinker with the mako template to get it to not validate your URL at all (presumably you trust yourself), and then package the result as a new custom module in your app. Beware that modules packaged in apps do not work correctly when running splunk on windows (fix is coming in 4.1.6)