Alternative uses for last five decimal places of an individual Bitcoin (0.000xxxxx) At the current conversion rates only the first three decimal points of an individual Bitcoin can be exchanged for a USD equivalent.  e.g. 0.002 BTC equals 2 USD cents.

That opens up possibilities for using the last five decimal places of a Bitcoin to include "metadata" about the transaction.

What alternative usages are there for these few bits and is anyone using them?
One idea that comes to mind is to use them as a transaction ID to bind the sender to the recipient instead of generating a unique Bitcoin address per sender.  If done carefully, this may allow more versatile use of vanity addresses.

Since the address space of the transactions is relatively small (only 5 digits to play with), one idea is to use a random 4 digit suffix that is valid for a short period of time.  Use the final / Fifth bit as a checksum that can also be ommitted if the conversion rate BTC to USD exceeds $100 but is below $999

Example


An item costs 10 BTC, and the buyer clicks "check out"
The seller creates a new transaction ID (9998) records it, and makes it expire in 1 day.
The seller calculates the checksum.  Since the sum of 9+9+9+8 is 35, an odd number, the checksum is set to "1".  (if it had been even the checksum would be "0")
On checkout the seller pays 10.00019998 BTC to the seller, possibly to a vanity address
The payment is received by the seller and visible in the transaction history.
The seller does checksum of the transaction (9998) with the checksum bit (1) and passes
The result is even so the transaction is valid (less likely a typo was made)
The seller looks up transaction ID 9998 in the database.  If the Tx is invalid, or the date is greater than 24 hours, then a refund is sent to the seller.