using alpha testing with shadow maps

While its trivial to enable alpha testing in a shadow mapping algorithim, the problem comes when you want to a floating point render target that dosent have an alpha channel, like the D3D format R32F. This format is usually faster than somehting like A16B16G16R16F. If the render taget has no alpha channel, you cant use any kind of alpha testing or alpha blending when rendering to it. Is there anyway around this, to fake the alpha testing while still using a format like R32F ?