I found it's easy to write your own plugin to parse the H.264 stream, locate the NAL unit, and insert SEI message in a dynamic way. Searching the google, and viewing Stackoverflow answers, don't give much clues. Even someone is selling a plugin for about 3000 dollars on the net. Oh Jesus!
In fact, the best way is to read the GStreamer official document about writing a basic plugin. And figure out your own one. It's a mystery why there are so many books on FFMPEG while so few on GStreamer. On Amazon, no book execept one user guide of GStreamer , which seems is exactly a copy of official docs. Though the official document is execellent, there is no reason there shouldn't exist a book, for dummy, receipies , head in ... series , anyway.
There are 2 classes in a plugin. One is "object class" , the other is "element class". Object Class is the base class of all plugins. Element Class is the instance class of the plugin.