How about placing fixed prices in the TP with bonds and m-boxes.
It would give you a chance to regulate the amount flowing through without players hypothetically leeching this system for extra GP.
In theory this may sound plausible, however allow me to explain why it would never work.
First off, this would require coding to make it so that the bonds and mystery boxes cannot be traded/dropped/looted. You would also need to make it available to be sold in the trading post, at a set price.
In doing this, you create a system in which players have to constantly refresh the TP, in hopes that they can buy a bond, or a system in which bonds do not sell because their set price is deemed to high by the player base.
The next issue is how iron man will receive ingame bonds, as the only current way is to use a bond on an iron man. If we remove the ability to transfer bonds and only use the trading post then you run into an issue there in regards to ironmen who want to use in game bonds for donor ranks. This could be fixed if ironen are given access to the trading post, but only for bonds..but this also requires even more coding.
Lastly, you're forced to add a set price in the trading post, as you cannot just have an npc who gives 30m~ for a bond, as that would flood the market with spawned gp essentially.
The stronger solution if you're looking to go this route is to force players to pay x amount in order to consume a bond (for example 1-3m per bond) @
You get the best of both worlds, you can somewhat make sure players lose profit, provide a gold sink, and avoid massive amounts of coding/and set prices.
By doing this you are incentivising the players who are spending real life $ for donator ranks, rather the players who are spending in game wealth to reach higher ranks. It's 2:30 AM so my ideology here could be off, I look forward to the responses in regards to this suggestion.
DISCLAIMER: As I mentioned in my previous posts, the economy adjusts based on things like these, so if we assume bonds are 30m ea, they might very well drop to 27m as a result. It's difficult for me to determine such cause and effects.