Illustrator :: Convert Greyscale To A Spot Colorscale?

I own a textile screen printing business. Sometimes I vectorise photos or images into greyscale. This gives nice halftone images, once I output to my printer through my RIP, (I use Accurip). This allows us to reproduce some great images with 1 ink colour. The great thing is that we can output the greyscale image, then use any ink colour we choose on the printing press to actually print the image. If we output greyscale, we can use scarlet ink to give us all shades of scarlets and pinks if we print onto a white shirt, for example. No problem there.
 
However, greyscale is essentially based on a black ink, with variable transparency. When I want to do a mockup for the customer of what the image would look like with a different base/ink colour other than black, (and the variable opacities of black that form the swatch pallette), I'd like to show the image using the actual base pantone spot colour we will use. That way, the customer can see how the variences in ink transparency will look on any shirt base colour that we choose to put in a layer behind the image. Right now, we can only show what it looks like if we choose black ink.
 
So..... once I have a vector in greyscale, how can I recolourise all the various opacities (of black), to various opacities of...say, Red?
  
If I try any kind of overlay or hue / saturation changes in photoshop, the base black colour is still present, and doesn't accurately represent what would print on a white shirt.