Here are a bunch of my thoughts, some conflicting with others.
Prologue: my overall opinion (or lack thereof)
First off, I'm also on the fence about this change. I would love to be able to have more access to iron to build hoppers, rails, armor, etc. I have an iron farm in my solo vanilla world for just that reason. However, I do understand the economic implications; iron is currently a very fair currency because it's relatively evenly distributed throughout the world, is relatively easy for new players to mine, and is of value to new and old players alike. This allows new players to start mining and almost immediately be able to buy meaningful amounts of things from players who have been around for ages.
Will "marking" farmed iron solve anything?
One of the ideas that's been going around is "marking" iron from golems and preventing it from being traded. I don't see how that would help. Currently, one of the reasons iron is a good currency is because it's useful. People sell for iron not only because they want to buy other things with that iron, but because they can build things with it. If players could farm iron, they would not need as much of it. Therefore the intrinsic value of iron drops, and the economy is affected nonetheless. Established players who have their own iron farms will be less willing to accept it as a currency because it's only useful for buying things from players without iron farms.
Alternative to "marking" iron from farms
In case my previous point turns out to be invalid, here's a variant on the idea that might be more intuitive. Rather than labeling farmed iron (and its crafted derivatives) and preventing them from being traded, it might be simpler to use iron *ore* as currency instead of iron *ingots*. I haven't thought this through completely, but here's how it might work. During a maintenance period, go through all inventories and convert all iron ingots to iron ore. Maybe some conversion of nuggets and blocks as well. Also change all shops that buy/sell iron ingots to buy/sell iron ore. Next, disallow iron ingots, nuggets, and blocks from being used as a trade item.