```   \input{MinionPro-FontDef.sty}
\Mn@DeclareLargeFontFamily{T1}{MinionPro-OsF}{osf}{t1}

But these macros rely on the keyval package

Does that really work in 2e?

fd files can be loaded pretty much anywhere, where the font is first
used, in particular in a local group. The commands that are designed to
go there all make global definitions internally.

Package files in general (and certainly keyval) assume that they are
going to be loaded at the top level and normally make their definitions
locally.

So either you load the package every time  which is a bit slow and can
be expensive if the package allocates any registers etc or you arrange