Just following up on this thread and found the following on 1Password’s forum dated 8/14/15 from Drew_AG - AgileBits Team Member.
"Thanks for taking the time to contact us about this! Although I’m not very familiar with DEVONthink or their proper browser called DEVONagent, my understanding is that they do not support extensions (even though they’re both based on WebKit). I wasn’t able to find information about that on their website, but you may be able to confirm that yourself by checking to see if you can install any extensions in that browser.
If they release a new version of their web browser that supports extensions, perhaps we’ll be able to offer a 1Password extension for it. But for now it looks like that isn’t possible. I’m sorry I don’t have a better answer for you about that! If you have more questions or need anything else, please do let us know."