should this be done in dlls/oem_drivers/ or so?
+1 from me, though I'd vote for a new top level directory,
thirdparty/
or somesuch where we can move DLLs that aren't a part of the core Windows OS but apps need anyway. There are some DLLs in dlls/ that are basically a mystery - for instance, monodbg.vxd. The comments say it was for Origin games but it appears that the implementor (was it marcus?) doesn't actually know what it's for either!
"thirdparty" is a bit of an awkward name. Maybe vendor?