I don’t understand what you mean.
What’s private in a public comment in a community tool like iNaturalist?
If comments were supposed to be private, then they would be private.
As for it being a common use case, of course it is not.
We are talking about using the API, which is uncommon by itself.
But in project scenarios where you want to compile observations using the API, doing the hard work of submitting them to your own team of experts might be not uncommon.
There may be many museum staff willing to collaborate in citizen science projects. But I bet the vast majority of them are not going to do it if that implies having to log into whatever, learning a new website interface, installing stuff into their mobile phones, and things like that.
On the other hand, if I email them a weekly worksheet report of already filtered and tabulated observations, they would certainly give me feedback introducing their identifications and mailing them back to me.
In that scenario, my daily work of tagging experts in observations’ comments would be a very productive way to keep things ready to be filtered and tabulated during the weekend.