There are a bunch of songs literally made for the game, as far as I know they're public domain with no content-ID, so game permission is good enough for those. But there are also built-in songs that the game has licensed, so they probably wouldn't be able to play those on stream, they'd have to be careful about which are which. They could probably get permissions for the Camellia pack.
For custom songs, there are a good number of high-quality Hololive song maps, but the people mapping them and providing them for free are actually violating Cover's copyright. So they'd be allowed to play them except that it would be like explicitly admitting Cover is ok with the copyright infringement, and that's probably not something they want to do.
The only way around that as far as I can think of would be to privately commission song maps themselves, for at least $300-$500 each.
And then there's mods. If the assumption that Cover needs to seek permission for every single mod they use is true...
For a decent view that won't make people sick to watch, you either need:
- LIV (seen in Pochi's vids), which is watermark-or-pay, but probably easy enough to secure permission for,
- or Camera2 + Custom Avatars (won't do full body tracking but that's probably not necessary), which depend on other mods, and would therefore require permissions from 6 individual mod creators.
Disclaimer, I am not a lawyer, just a sperg. All of the above information may be incorrect, but I said it confidently, so you can believe it anyway.