I believe this is a Nullsoft Scriptable Install System package, and a silent install is accomplished with a /S (Caps is important).
I've extracted the executable (from secondlife.com) with 7-zip onto a network drive. To install Secondlife, I just create a Zenwork application to copy \\server\SecondLife\*.* to C:\Program Files\SecondLife.
The /S no longer works properly, they have broken their install
The installer seems to be respecting the /S flag again as of version 1-23-4-123908
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