Are there any existing libraries that will allow an Android tablet (4.0.4) without an NFC chip to communicate with an external USB NFC reader / writer?
Yes, the ACR 122 U can be connected via an OTG USB cable. ACR has an Android testing app on the page. In addition, you will need something like NFC Tools for Java to interact with the reader (send commands, etc.).
The ACR 1222L reader has an android library.
http://android.acs.com.hk/
The documentation for these libraries can be found here:
http://www.acs.com.hk/download-driver-unified/5102/ACS-EVK-Android-112-A.zip
Perhaps the default packages will help you. Otherwise, you can search for any third-party libraries.
http://developer.android.com/reference/android/nfc/package-summary.html
or it can help you
How to enable NFC reader via API?