First off: Thank you for your contribution!

Matching the p7zip version sounds plausible (but how is that version "defined", i.e., why is it 17.7? Hum)

Besides that, would it perhaps make sense to also match the name more closely? E.g. p7zip_alone2 or something like that? Otherwise it's really confusing: I'd immediately wonder what the relation between p7zip and sevenzip_standalone is.

To that end, a comment in the build recipe added here (and ideally also in the original p7zip recipes) explaining the relationship would be helpful, I think.

The name was from the Debian package https://packages.debian.org/trixie/7zip-standalone, but 7zip-standalone wouldn't work because it starts with a number and has a dash. Maybe p7zip_full would be better? Though on Debian https://packages.debian.org/trixie/p7zip-full refers to the version of 7zip that dynamically links to a 7z.so

Should Alone2 somehow be in the package name?

Perhaps good for @giordano to take a look here.

Yeah, doesn't look like this is going to happen in Julia

I'm currently happy with just using the formats supported by the p7zip_jll package, so I'm closing this PR for now. But feel free to reopen, or make a new PR, if you want to use the less common formats in alone2. I'm also not sure if the implementations in 7zip are high quality or well tested for those less common formats.