For context, I am testing the use of SystemVerilog plugin at work. I submitted an issue last week to that package's bitbucket issue tracker, and to their credit they fixed the issue within days. I think, however, that the change may have inadvertently broken something else.
I'd like to roll back to the previous version of the package. Is that possible to do from within the Package Control plugin? If not, can I roll back manually (ie, from source)?
If the plugin is being kept under source control (and it sounds like it is), then you can manually install whichever version you want from the source control server - I don't use Mercurial and/or Bitbucket, so I can't give you the exact instructions, but I'm sure you can find them. However, Package Control does not have the capability to install arbitrary versions, that I'm aware of.
The way it works is it fetches the latest version number from the package's repository, keeping track of the most recently released version as well. It does this about once an hour. When an Install command is issued, Package Control requests the version in its database from the repository. If the version number increases, then Package Control pushes out an upgrade. However, like I said, it doesn't have the ability to go back in time - you'll need to do that from the source control server itself.
As an aside, why bother downgrading? If the developer is as responsive as you say, just submit another issue, linking to the first one, and see if it gets fixed quickly. No need to keep track of versions and manual vs. Package Control installs, etc.