Silk Screen Image that covers vias

lol, nice to hear someone had the same ideas

I actually did make some boards with different soldermasks through two exports
looks fine,
works well if you know what you are doing.

1. 
   for silkscreen design larger than the board size, I think fritzing cut them off while exporting.

or if you mean combining two different size board's silkscreen for output,
no, I don't know the answer....