The proposed attribute could be applied to numeric properties of classes that are deserialized from Bson Documents It shall allow you to become more strict or less strict about:
gaining precision
losing precision
gaining the ability to have a fractional part
losing a fractional part
going from float to decimal
going from decimal to float (if BSON is amended to allow a fixed point decimal)

I'll propose the name BsonNumericConversionAttribute, but I am open to a shorter name if it does not reduce the clarity of what it does.