This is a simplistic Lua script which uses a command pulled from chat text to enable derendering avatars rezzing after the command is invoked. It leverages the callbacks OnSendChat() and OnAvatarRezzing() via Cool VL Viewer's Lua scripting as defined
here. Once loaded, it is invoked with /da [on|off]. Passing no argument displays the current state.
When enabled, all newly rezzing avatars (except friends) will be requested to derender via DerenderObject(). An array (Lua table) will hold the ids of derendered avatars, such that when the function is disabled, the viewer can be instructed to allow rendering again for those avatars when a server update occurs for them. Note: avatars will not derender/rerender immediately. See the documentation listed above for details.
I make no warranties for this script and it is a 'use at your own risk'. The enabled state is not preserved between viewer restarts or indeed Lua script reloads. I have extracted the attached script from my larger, master script. Feel free to use and alter as you will. I will try to answer questions related to it.
Enjoy!