Color Replace Shader Unity2D

Hi,

Im building a 2D RTS game using Unity 4.3. I need a way of colouring the units based on which team they belong to.

The way I had envisioned doing it was to have a separate texture per unit that masks a certain part of the unit and colours it to whatever colour is that team.

So for example the unit is:

Then if I wanted to colour her hair then the mask would be:

Then in a shader I would do something like:

 fixed4 frag(v2f IN) : COLOR
 {
     half4 original = tex2D (_MainTex, IN.texcoord);
     half4 greyscale = half4 (original);
     greyscale.rgb = dot(greyscale.rgb, float3(0.3, 0.59, 0.11));
 
     half4 maskedPart = greyscale * tex2D(_MaskTex, IN.texcoord1).a * _ReplaceColor;
     half4 unmakedPart = original * (1-tex2D(_MaskTex, IN.texcoord1).a);
     return maskedPart + unmakedPart;
 }

Which will then only tint the hair whatever colour the team is set to.

The problem is that I am unsure how to pass in the extra texture coordinates (IN.texcoord1) as my _MaskTex may be laid out different to _MainTex and thus the per vertex texture coords will be off.

The only way I can think of doing it is by totally rewriting Unity's SpriteRenderer as its a sealed class and cannot be extended :(

Does anyone have any suggestions?