Interesting concept.
It is a bit like my travel card - I feed it pounds and it returns journeys. It could definitely work well.
However, I do wonder whether this is all adding complexity to circumvent a problem of having currency units which are too large. If we just had smaller units to start with, we could just have an integer representing network credits on your account, which map directly to safecoins.
While I understand that having units which are very small also makes trading them a bit unwieldy, we can group these smaller denominations together and call them larger denomination names (e.g. satoshis vs Bitcoin, but on the other side of the decimal place)
I also understand it could make transfers more chatty as transactions would be made up of many micro-penny sized transfers. However, this may be a price worth paying for simplicity, especially as it would give an easy ‘out of the box’ solution for micro payments.
We will have a single opportunity to set the denomination size at the start and everything beyond that point will have to work around this. I think we need to be very careful not to create a rod for our own backs.