As mentioned in that thread, you can use the same preview handler that Explorer uses for video files in Opus.
This can be done by disabling the Opus Movie plugin. It will then fall back on the ActiveX+Preview+Office+Web plugin, which will use either Windows Media Player's ActiveX control (the default) or the preview handler (can be set via configuring the plugin). The main difference between the two is the former autoplays while the latter is click-to-play.
You mentioned that Explorer isn't showing AVI files on your system. That would usually be due to a missing codec or splitter.
But I request for update Opus player, not using Windows Media Player. It's faster to use internal player than external, even if it's Windows Media Player. Is this really that big problem to use proper modern renderer in DO12 built-in player?
But, sorry for question, why not update your player? Is it really that hard to make one extra option in DO? I though that if every modern video player has option to choose video renderer, Opus should have it too.
In the meantime, it doesn't happen by itself, there are a lot of other things people are asking us to do, the current player works well enough (literally no one has ever complained about this before in the history of the forum), there are already alternative players you can use if you don't like the default one, and there's a plugin API which you can use to write your own video player if you really don't like any of the options on offer and are so sure that it's a trivial amount of effort to write a new video player. Plenty of options there for you!
I didn't know that change video renderer is the same as write new video player. If it's that difficult then ok. But still - so many years, other players change that already (you can find some complains on VLC forums for example, some years ago - fixed now of course).
If is that hard work (not just change renderer or if change renderer is so complex work) then I'm just don't know that. Is really just idea to make opus more... new standards compatible? I don't know, just useful.