I do this every 4 months, recolor wmf's for a 2 spot color job, it's driving up the blood pressure right now.
I have saved a two color swatch set as ai set (not ase set yet) to the default user swatches folder and the client job folder, Black (cmyk default swatch) & Relex Blue>spot (It shows the spot icon, circle, cmyk mode).
I drag in or open a wmf, (color mode rgb, I haven't seen a requirement to change doc color mode to cmyk, but do that first). Then, with the object selected, Edit > Edit Colors > Edit with Preset > 2 color Job > Choose a Library to use with this preset....I have the swatches included in doc swatches along with the open color library but Ill replies that there are not enough solid color swatches available.
What am I missing here, the spot Reflex is not a tint (near as I can tell) nor a gradient.
Have you tried by declaring the Black as a spot color?