Good idea, should fatten the unison sound nicely. One slight caveat: it will only work with chromatically sampled instruments. Most VSL sounds are chromatically sampled, but their Special Edition instruments are sampled at every tone, I think.
>Pitching half a tone sounds easy
It is easy - in the VI's PERFORM window, set the pitch bend range amount to 100 cents (=one semitone) and push the pitch wheel all the way up.
>The pitch value goes back to zero, whenever I stop playing a song.
In any sequencer, if you write a pitch bend command at the top of a sequence the pitch will stay 'bent' during the duration of the sequence. In Logic, I think it stays bent even when you stop the sequence.