The reason I read at the devs is, that in the Android environment permitting apps to execute from external/removable storage is considered a serious security risk.

"But, but, but" I hear, "NativeBOINC does", and it seems they applied a serious hack to get it to do that... just that u know!

So, until the Berkeley devs find a legal way, IBM [those behind WCG] gives the okay, they do code auditing of client too, to use the SD for storage, yet load the apps from internal memory, it's going to be a no-fly zone.