It would be great to be able to "boost" posts/threads from across the Fediverse (and in particular, topics from other forums) into a local NodeBB category.

Is there a way to do that currently, or a likelihood of it being added it in the future?

Yes! You can do this already, actually. It's a bit of a hidden feature because I only added it fairly recently.

Remote content when retrieved by NodeBB are automatically slotted into topics as best as we can contextualize them. These topics are stored in a pseudo-category called "Uncategorized" (more information about that pseudo category in our docs.)

You can't browse to cid -1 directly, but the closest you can get is:

• going to /world, which is a cross-section of that cid from users you follow, or
• going to /recent?cid=-1, which shows a reverse chronological list of topics

When as an admin you use the topic tools to move those topics into an actual category, NodeBB will federate out an Announce, which is what you called a "boost". The original topic author will be notified of the announce, and it will show up on their end as a "boost"/"reblog"/etc.

Ahhh! So (ignoring the discovery aspect) if I were to paste a remote post URL into the search and open it like that, then "move" it into a local category, that would work as a boost? I'll have to try that!

And yes, it works!
It's certainly unintuitive but that's fair enough for a work in progress!

It doesn't appear in the recent posts for that category (it was an empty category and still says "no new posts"), but that's a minor detail.