The kinda bad news

Since d64e2df I was hoping that tooltip icons could finally be made to work in REI as well.

So I tried and this is the best I got so far:

What I did is ripping out the ol' enchantedItemStack which I believe is completely superfluous now and was blocking the tooltip logic for no reason - but as you can see, instead of tooltip icons it drew "This is not supposed to be used as an actual string.", this is because REI thinks the tooltip is pure text and converts it to a string.

What you see at the bottom left is the result of adding a tooltip component (added similarly to shedaniel/RoughlyEnoughItems#568) which isn't considered text, but this has the drawback of always being rendered at the bottom and as you can see, I couldn't get it to actually render on top of the tooltip (I offset it to be able to see it), despite trying all ways of changing the zOffset or actual z value.

You can find this code on my experimental branch.

The good news!

So while looking up the references to the relevant GitHub issues and REI code just now, I found this:
shedaniel/RoughlyEnoughItems#645

Apparently the AppleSkin author a very similar issue and there now seems to be an API for this!