Curious if you'd consider this, or possibly there's an existing solution already.

We're using Puffish Skills to build out a large modpack skill tree, the plan is to have several different blocks/items locked behind each skill. It would be useful to have an items or blocks property in the definitions to specify an array of block/item IDs to be displayed (likely on shift?)

It should be possible to implement, but I think this is out of scope for this mod. Maybe there already is a mod that extends vanilla JSON text format and allows displaying items or blocks.

Looked around a bit and didn't find anything particularly promising for extending the JSON text object. The thought originally popped into my head from how Jade handles items inside a chest (see screenshot), but looking into their code it looks like they're rendering a fully custom GUI object.

I'll dig into extending the text object myself, we're going to be running a core mod in the pack anyway. If I find a good solution I'll let you know if you keep the issue open :)