I've been working on this for quite some time and finally got it working. It's a bit involved and requires some terminal use, but I can confirm I have WebTools working and it installs channels just fine. I haven't tested many and of course YMMV.
Part 1: Enable SSH access on your Seagate Personal Cloud (SPC):
1. Enter your device url in a browser with the following extension
?appdev=1. Example: http://192.168.1.100/?appdev=1
2. Select "Services" from the navigation bar to the left.
3. Second from the bottom should now show "SSH Access". Enable it.
Part 2: Find and enter your Plex Media Server Plug-ins folder:
1. Using your terminal, ssh into your SPC using your administrator (or first) account. Example: ssh email@example.com
2. You'll be asked for your normal password.
3. Go back one level from the default folder using this command:
4. My path to my plug-ins folder was this
/media/internal_1/rainbow/plexmediaserver-x3op672.data/plex_conf/Library/Application Support/Plex Media Server/Plug-ins. I'm assuming yours will be somewhat similar. Remember to use quotes around folder names that have spaces within their names. Alternatively you can escape them.
5. @"firstname.lastname@example.org" reported that the permissions on the "Plug-ins" folder needed to be changed using
sudo chmod -i which, to my understanding, causes the "Plug-ins" folder to inherit the permissions of the containing folder.
Part 3: Get WebTools (or whatever bundle you want to install):
3. Type this command to install WebTools:
sudo wget https://github.com/ukdtom/WebTools.bundle/releases/download/3.0.0/WebTools.bundle.zip
4. After it downloads, type this command:
sudo unzip WebTools.bundle.zip
5. To clean up the leftover zip file, type this command:
sudo rm WebTools.bundle.zip
Part 4: Restart your SPC and wait a long time for Plex to completely start.
Even after PMS was completely up and running, WebTools took a bit longer to show up under "Channels". You might have to wait, but it will eventually show.
That's it! I installed a few channels via WebTools and they seem to work. I'm sure there are ways to consolidate a few commands but I'm not very good with the terminal. Let me know how it works for you as I may have left a step out and I'll do my best to help.
Edit: I forgot to mention that I haven't updated my firmware that just came out. This works for now but may not with the updated firmware.
Edit 2: Added a possibly helpful step in Part 2.
Edit 3: Updated the firmware to 126.96.36.199. I can still access the Plug-ins directory.
Edit 4: Updated the firmware to 188.8.131.52. And updated the WebTools link to reflect version 3.0.0. It all works just fine.
Edit 5: Added step 5 to Part 2 to include a possible step.