Alpha-mapped terrain textures - problem

[Fobbah]

I'm struggling a bit here, basically my problem is that i need to be able to use smallish repeating textures across the terrain, utilising an alpha map that is much larger than this texture extending across the entire terrain..

The difficulty is in that i really figure out how to use an alpha map for this that is of different dimensions to the terrain texture, any ideas?  :-/

a simple implementation that does this and draws the repeating alpha mapped texture over a quad or something would be fantastic