I've added a property for the next NetTalk 5 build called
.MetaHeaders
Set this (typically in WebHandler, ProcessLink method, before parent call) to whatever you like, and this will get added to the <head> part of the HTML.
For example
self.MetaHeaders = '<meta content="IE=EmulateIE8" http-equiv="X-UA-Compatible"/>' & '<13,10>
Cheers
Bruce