If there's code that's widely available but not open source, anyone know any rough guideline on what can be legally "scraped" from that code in terms of its structure for use in an open source project? Are method and parameter signatures fair game? What about the names of methods/parameters?

in what form is it widely available? does it have a copyright notice? Is there any kind of license for it?

O'Reilly books on programming that I've seen have some rough guidelines at the beginning on what you can and cannot do with the code in the book.

but those notes only apply to the code in those books, not generally for any code anywhere.

good point, I should probably read all of the usage agreement (it's this http://interactivebrokers.github.io/)

I have a hard time imagining that anyone is going to have much to go after an open source project, if they copy the method signatures of a few dozen methods, but IANAL and don't know relevant case law.

3.3. You agree not to publish, disseminate, or redistribute the API Code to any third party.

yeah I might be being paranoid, I don't think it would really be against their intended usage necessarily

That sentence you quoted would make me a little nervous, depending upon what "the API Code" includes.

yeah it's basically defined as "all the code"

Sounds like this might be relevant: https://en.wikipedia.org/wiki/Oracle_America,_Inc._v._Google,_Inc.