For Linux users there is a little Go app here that works:
https://github.com/tyndyll/itch-claim
I've taken a look at the source and nothing looks sketchy.
If you trust the pre-compiled binaries, you can download them here:
https://github.com/tyndyll/itch-claim/releases/tag/v1.0
There are macOS and Windows binaries too.
To build from source, you need git and go installed, then:
$ git clone https://github.com/tyndyll/itch-claim $ cd itch-claim $ go build . $ ./itch-claim --username <your-username> --password <your-password> --url <your-unique-bundle-url>
It appears to add one game every 5 seconds. This doesn't seem like an unreasonable load to put on the itch.io servers.