How about this... Continue using LONG data type for amount.  As unsigned, this allows from 0 - 18446744073709551616 and we could always IMPLY 9 decimal points, such that now we have amounts from 0.0 NEM - 18,446,744,073.709551616 NEM.  This is well within the 4 billion limit.  The only thing here is we may want to do additions of volume, assets exchanges, etc such that that may not be a large enough number for appropriate representation.

In this case, perhaps we could just standardize on 5 decimals?  With 4 Billion, surely 5 would be enough decimals, giving us 0.0 NEM to 184,467,440,737,095.51616 NEM