Wouldn't a decent way to handle this be to add an optional units parameter?
Especially if the need for mBTC could be not too far off?
e.g., instead of
If units= is not specified, then the default of units=btc would be assumed.
.... it is how science got around the mess it found itself in in the early 1800's before Faraday sorted it out, implied/inferred unit conventions are a recipe for disaster, regardless of the context.
Specify the units and be done with it.
Edit: actually there is another incentive to explicitly specify units, a future use for the protocol maybe to send currencies other than BTC to a bitcoin account, e.g. dollars, euros, Goldau, could be possible future unit options ... (would have to be a nifty app. to do the conversion, interface with intermediary, etc, but you never know).