Apparently it was a multi-part bug - although shader_replace_simple works if you import the demo into 2.3, some issue was preventing macros (which are responsible for performing pre/post operations around shader_set/reset) from working when imported into a project with other extensions.
I have now released a new version that moves macros into a script and also a new version of GMLive that makes the server less confused about macros pointing at a script inside an extension.