Sacha,
I brought several Begonias over for the nursery to try:

'Little Brother Montgomery' - Maple like, silvered leaves pinkish when young, ageing to silver and maroon.
'Connee Boswell' - Large, maple-like silver-pink leaves. larger than LBM
'Benitochiba' - Interestingly divided, metallic, silvery purple leaves and pewter veins.
grandis 'Sapporo' - Coppery red young growths, taller than grandis proper and coppery leaf undersides.