The way I implemented it in multiAVCHD it may be possible to replace menu background (the static picture) with any background - just the way you mentioned DVD menus or replacing/editing images in BD-J.
I'll give it a try later and will post back... I didn't think of it until now (muxing h264 stream with an existing m2ts file)
Furthermore, it is possible to extract the MENU, export as BMP, edit and reimport in the menu m2ts.
On the other hand, it is possible to automate your GUIDE with a simple tool that will do everything that you described.
edit: btw, why do you need avisynth?