This is very promising. Although the previously posted Gamestudio demo has some lacks (starter window and the engine window is visible for some frames before it gets embedded), I think this could be a possible solution.

I was first disappointed that the whole game has to be download in advance but after reading the SDK help file of the OSA kit this is not necessary with the professional version. You can communicate with the browser and download files in another task and get percentage information.

To hit the nail: virtually, you can just tie together all binaries (exe, acknex.dll, plugins, etc.) and download nearly everything ingame, before you use it. Therefore you can log for each level all needed files and a custom level loader could check the list against possibly existant files on the HDD and otherwise it downloads it. When all needed files are downloaded, a scene will be loaded the usual way with level_load. Music should be nevertheless treated in a special way, I would suggest real streaming through FMOD or else. Even if a music file is only 3-4 MB, not everyone has a high bandwith internet connection

The pricing is okay. Though, it is interesting how long it takes until a competing web player for A7 is ready.