If you’re just using a keyboard driver, you can put a bluetooth dongle in between (if the phone supports wireless keyboards). Something like this perhaps: http://handheldsci.com/kb/
Though security is questionable, however, it is already questionable when using keyboard emulators: it’s not too hard to scan a tag without the carrier noticing it is being scanned, and log in to the client by typing the tag value.
Normally, the link between the tag and the scanner is secured (that is, there’s a security algorithm that enforces the tag can’t just be cloned). But if you just use the tag value, it isn’t a secure login anymore.
It should be fine for applications that don’t require high security though (i.e. not accessible to the public, and where you trust employees).