I'm am not a lawyer, I am not your lawyer, and if you take this as legal advice, you're foolish. Talk to a lawyer (although I suspect you're just being provocative)
No. That's not how it works. My layman's understanding is if I make a change to a Magento source code file, and then distribute that change to another individual or organization, that individual or organization has full rights to that change under the OSL 3.0. I can't restrict access to that code under copyright or a commercial license once I've given it to them.
So you, as a third party, have no rights to that change since it hasen't been distributed to you. If you could convince the store owner to give you a copy, or convince me to give you a copy, you'd have full rights under the OSL.
There's also a huge grey area of IP law around "plugin" or "extension" code for open source software systems. There's been a few cases in the past where platform owners have tried to claim plugins and extensions, even if they contained 100% original code, fall under the same license as the software they're plugging into or extending. This has never been tested in court, and usually the platform owner backs down and simply stops promoting plugins or extensions that don't conform to the license they want.
This happened with the Joomla community and the GPL. In the end, Joomla stopped accepting plugins to their extension directory that didn't use the GPL license. This was never tested in court.
Since March 31 2009 the Joomla! Extensions Directory (JED) no longer accept non GPL and since July 1 2009 only GPL extensions are allowed to be listed here
It also flared up in the Wordpress community over the Thesis theme and the GPL. This was again made moot, but this time because the Thesis theme had actually copy and pasted significant portions of code directly from the GPL licensed Wordpress code. This made it crystal clear that the Thesis PHP file needed to be distributed under the GPL.
Standard disclaimers about IP law growing increasingly less clear cut, and being more about "what are you willing to press charges over"