Transaction at timestamp and managed mode

I want to be able to create read-only transactions at some timestamp of my choice (NewTransactionAt). It is only available in managed mode. I was wondering if there is a way to make managed mode generate commit timestamps automatically. At first, I thought that I could just have an atomic counter for that, but then I saw that I need to wait for all in-process commits like it’s done here. Can I have something like that done automatically, or I should write something similar to this watermark mechanism that oracle has?

I think my question can be reformulated in a more simple way – is there a way to create read-only transactions at some timestamp of my choice in non-managed mode? I don’t see how can it break consistency, but there is no way to do it right now