Hello there @taylorlorenz@mastodon.social,
I'm not too familiar with Substack, but a short look at “Substack Notes” makes it seem like it's a microblogging feature similar to Mastodon. I think that could easily be part of the Fediverse. As a matter of fact, considering that the “default incoming maximum post character limit” for a Mastodon server is ~100,000, even regular Substack Blogs could be federated without any hurdles or display issues.

There is a plug-in for WordPress called “ActivityPub” which turns every WordPress blog into an ActivityPub enabled server. If you wish to compare it, it's like turning the RSS feed feature on or off.

If your WordPress Blog has comments enabled, ActivityPub replies could be displayed as comments. A Substack Blog could do the same, Substack Notes could basically be a microblogging platform akin to Mastodon.

I believe it could be federated, it just takes more work. Next to the general implementation of ActivityPub, which has its hurdles, now that anyone from across the interweb could write a comment, would you need an additional spam filter? Should federated posts be displayed on their platform too? What advantages does Substack have for opening up their platform (next to being good for their publishers and users)? So maybe just publish Notes and Blogs to the Fediverse but don't display any interaction? Some of these questions may be what hinders Substack from doing this. Technically, it would be doable, how easy it is I cannot judge as I don't know Substack's code. Taking Ghost or the aforementioned ActivityPub plug-in as an example, their implementation of making blog posts federated was quite simple and didn't take longer than a few months (the ActivityPub plug-in is open source, its code doesn't seem too complex, especially considering it's merely a plug-in and nothing that changes WordPress fundamentally)...

TL;DR: Yes, Substack Notes could be federated, same for Substack Blogs.