Make lightmapped object transparent

For our game we need the ability to change prefabs/gameobjects semi-transparent. We first used the Transparent/Diffuse shader and set the color with alpha in a script attached to the object.

However, for performance issues we need to switch from realtime lights to baked lighting. It actually looks better that way anyway

The problem is that the solution above no longer works. I tried some of the transparent/lightmap custom shaders you can found around the forums but they require you to set the lightmap texture and tiling/offset params which is quite some work as Beast already generates those. Is there a way of using the internal shader with lightmaps and transparency?