How do you define transaction cost? Does it include risk, literal fees, confirmation time, convenience?
All of the above and more.
Mt. Gox codes present a default risk, but there are no fees, transactions are nearly instantaneous, and at least where accepted, they are convenient. They are essentially one-time use and could not replace bitcoin because of the redemption race condition.
Mt. Gox codes, however, are not in general accepted instead of Bitcoins as a method of exchange.
But I don't see that it follows, as you seem to imply, that such instruments do not inflate the money supply even if only minusculely.
The reason why such instruments do not increase the money supply is the same that deciding to buy an apple instead of an (originally desired) orange does not increase the supply of oranges: for people other than those involved in the transaction, they do not, in general, act as substitutes.
During the short life time of a redeemable code, before it is redeemed, doesn't it carry the same transactional value of a static bitcoin that might otherwise have been used?
What you describe is a reduction of the size of the Bitcoin economy and an increase of the size of the "Mt.Gox Bitcoin code" economy, rather than an increase of the money supply (of Bitcoins).
Is it any different if the US Fed doubles the paper dollar supply in the morning and destroys the excess in the evening, versus some other doubled credit instrument with a twelve hour lifespan? I understand the stickiness of prices (wages probably won't budge nor will prices double), but when a street market vendor notices that his fresh fruit are selling especially quickly, might he not consider raising prices or pulling the discounts?
Paper money and demand deposits (or another credit instrument) in fiat act as substitutes (of each other). A necessary condition for this is that demand deposits (via cheques, ATMs, EFT) often reduce transaction costs compared to paper money. Mt. Gox codes denominated in Bitcoin do not (and, possibly, never will) reduce transaction costs, and are not, in general, accepted in Bitcoin-denominated transactions.
I could believe that digital money increases velocity by design, providing an extra inflation for which gold is not similarly susceptible.
Money velocity and money supply are two different variables.
And I suggest you read more in-depth books about money supply, it does not matter whether you choose Keynesian, monetarist or Austrian, since they all support my position.
A bank can not print paper dollars, euros, yuan, but they can create M2 credit (aka money) just as I can create M2 bitcoins. A fractional reserve bitcoin bank can get into just as much trouble as a traditional bank. The only difference is that no one lender of last resort can bail out a bitcoin bank through monetization (printing).
The definition of M2 as
quasi-money includes that a redemption of instruments belonging to M2 increases M1. For redemption of fractional reserve M2 instruments to increase M1 this requires that there exist fractional reserve M1 instruments. With Bitcoin, they do not (even though they are hypothetically thinkable, if there was a M1 instrument that decreased transaction costs, such as Ripple).
Let me repeat that: the existence of fractional reserve M1 instrument is a necessary condition for Bitcoin-denominated debt instruments to affect the money supply of Bitcoins.
I can take bitcoin deposits from all of you and lend virtual bitcoins.
But unless someone else than the loan taker accepts these virtual bitcoins as if they were real Bitcoins, this would not affect the money supply. Conversely, unless people in general accepted these virtual bitcoins in the first place, the loan taker would not accept them either, and your business model would not work.
As long as I keep up a confident smile and not all of you simultaneously run to withdraw, I can keep the ponzi scheme fractional reserve system in perpetuity.
And the reason why this would be a pyramid scheme is that there would be no other use for these virtual bitcoins than to get people to give you real bitcoins.