The current version of the port did not include support for serial (ISDV4) tablets, such as the ones built into Thinkpad x200 and the like. The original code does support them, so after removing several linuxisms they work on FreeBSd too.
Additionally, the isdv4-serial-debugger application is now built and installed into the system (again, it was disabled in the current version of the port because of linuxisms).
NOTE: This patch can easily be applied on top of ports/175876 once that is committed.
Also note that I haven't tried updating to a newer version of xf86-input-wacom (0.22.1 at the time of writing) since there are quite a few makefile changes, and I am not well-versed in makefiles.
Fix: Patch attached with submission follows:
How-To-Repeat: Try building the current version of the port and find that X server does not detect a serial wacom table (provided you have one).
$ cd ports/x11-drivers/input-wacom
$ svn patch --strip 1 /path/to/input-wacom-serial.diff
And then the tablet should be detected by X.
Committed in the new x11-drivers/xf86-input-wacom