The same with us (that's why a rewrite of h264 is not top prio). i just looked up our figures for video deliveries for our production site: factor h264/html5 = 1/9, with peaks on htm5: 5.500 per hour.
Concerning ad_returnfile_background: it has the same interface like ns_returnfile. The changes for NaviServer in OpenACS are mostly due to using features of NaviServer not available in aolserver. If you get a recent version of OpenACS (e.g. the tar file), just grep for NaviServer, these should be pretty easy applicable for every OpenACS mutant.
