OK if you talk "loop points", I now start to think you mean you have an initial part of the sample which is unlooped, then a range which upon entering you want to loop for the rest of time.
Pretty sure that's what the 3rd playmode in soundforge is all about but it's pretty rarely used in any file but I do think you are right that it's an existing concept in RIFF.
[edit: Removed reference to ADPCM. I am muddling my shit. That is a lossy format that can fit in RIFF container which definitely does have loop points. Less sure on genuine PCM].
I'd hope foobar2000 may be aware but "frog knows", I'm afraid.
edit: Also, I don't suppose you've tried Winamp? That's the kind of thing that is more likely to be aware of such features than the big boys [2.91 was one of the "good" old versions from memory. I get a feeling you may have more luck with older things than newer, as everyone kind of moved away from "small looping samples" toward entire tracks in PCM as hardware resources grew]
Finally, also, if you are deciding your own loop points, you may need to do a tiny fade in at the beginning & fade out at the end [plus a fade out right at the end of the unlooped portion right before the beginning] - I mean only around 5-10 samples, to ensure a zero crossing.
Final final edit, I promise. This sounds as if Winamp had a plugin and foobar2000 also does:
https://languor.us/looping-audio-track-foobar2000
Unsure of native WAV support from anything..
