I don't believe that there is a Windows 7 Upgrade OEM. The upgrade disk that your OEM sends you will probably be a retail upgrade disk. And really, Microsoft's only requirement for an upgrade is that the prior OS is Genuine and has a COA (certificate of Authenticity) from a licensing standpoint.
So... I assume your OEM is giving you a retail upgrade disk, and unless they make a custom build OEM upgrade disk that checks your machine to see if it is in fact an OEM machine (Dell, HP, Lenovo, etc...) I don't see how they can regulate it. I am not even sure that it would fall outside of their end user agreement. Yes they are sending you an upgrade free for purchasing their hardware now, but not sure that upgrade is force to be on that hardware over any other Genuine Windows System.
All purely speculation though, you would have to read the OEM's upgrade policy when applying for the upgrade.
BTW, what OEM?