It is possible, but the downside is that it would be relatively slow to do. The root of the spline typically has almost zero radius corners, so a very small ball mill would be required.
If you can acquire a taper endmill with a small enough tip diameter to fit in the tooth space, and a taper which matches the pressure angle of the spline specification, you could likely generate a close enough approximation of the involute in 3 to 6 passes down each side of each tooth. However, this may not be the most friendly style of tool to surface a cad surface model with. Nonetheless, through a combination of rotation of the work, coupled with a lateral offset of the taper tool, it would be possible to generate a curve and get down into the root of the tooth space to cut away those troublesome last bits.