Best codec for playing unfinished files with VLC
You can use H.264 in MPEG-TS.

ffmpeg -i in -c:v libx264 out.ts


In Potplayer, I can play as well as seek. In VLC, as well, but timestamps are missing. Limitation is that you can only seek to keyframes. If you select a non-keyframe, then the video output will be frozen or corrupted till the player reaches a keyframe. Audio will work in any case.