Switching directly through multiple keyboards

Hello !

A friend of mine, quite involved in ancient greek, sanscrit and phonetic
alphabet has defined multiple xkb keyboards file. He wanted to know if
there is a way to shift directly to a particular definition, rather than
cycling through 3 or 4 different ones. 

To express it more "factually", is there a way to assign, let's say
SHIFT+SHIFT to a greek polytonic kbd, SHIFT+ALT to sanscrit, SHIFT+META
to phonetics, etc.

Thanks !
Vincent