Someone had asked this a while ago on one of the mailing lists...
The only way (AFAIK) to pass arguments to a WS Lua script is to use environment variables, as shown here:
$ echo "print('DEBUG', os.getenv('DEBUG'))" > test.lua
$ DEBUG=1 tshark -v -Xlua_script:test.lua
TShark 1.7.1 (SVN Rev 39998 from /trunk)
Copyright 1998-2011 Gerald Combs <firstname.lastname@example.org> and contributors.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Compiled (64-bit) with GLib 2.28.8, with libpcap 1.1.1, with libz 1.2.5, without
POSIX capabilities, with SMI 0.4.8, with c-ares 1.7.4, with Lua 5.1, with Python
2.7.1, with GnuTLS 2.8.6, with Gcrypt 1.5.0, with MIT Kerberos, with GeoIP.
Running on Mac OS 10.7.2 (Darwin 11.2.0), with locale en_US.UTF-8, with libpcap
version 1.1.1, with libz 1.2.5.
Built using llvm-gcc 4.2.1 (Based on Apple Inc. build 5658) (LLVM build
07 Dec '11, 13:27