What you guys are suggesting is absolutely similar.
Not quite. In this case, it's something that costs MA money, unlike the Arkoins for Arkadia. To them, getting rid of X amount of deeds will give them more income/savings than a straight PED amount in a sale without even having to spend PED to buy back the deeds (let's assume no costs associated with developing the castles for now this time).
So if MA gets say 1 CLD for something, the deed alone has no TT value and MU is irrelevant to them unless they were paying to buy the deed. That's the key thing. They could set a sort of auction minimum to essentially say X deeds = 1 castle. It doesn't matter what the MU is because the conversion rate is at least X + whatever additional bids are. At that point, the exchange to MA is worth whatever the payouts on the CLDs are for their entire lifetime without paying anything to get them.
So let's say MA sold a castle for 150k PED direct (~worth of 75 CLD) instead. What then happens is if they wanted to buy CLDs, they would be reliant on people first selling without incentive, but they'd also have to deal with the potential for MU increasing while doing that.
Creating an exchange for a unique item instead let's them potentially spur the first, and control the second (i.e., give us this deed that has no immediate value to us, but has future payouts, for the castle that has no immediate value to us). The latter one is probably the bigger thing though. At that point the economics of PED doesn't matter as much because a unique exchange has been setup for something else. It's true if they lowball that exchange rate and don't value the "lure" effect of getting deeds not on the market appropriately compared to maybe getting more from a direct sale, they wouldn't be better off with the CLD route. That's part of the figuring they'd have to do though, especially since there's a bit of an apples or oranges comparison in accounting for that lure effect, so it's not easy to really total up all the unique added values from doing a CLD exchange.
This is all why valuation with barter and trade is really complicated when a currency isn't directly involved in the exchange (once-removed in this case). Some of the very things you mention about availability play in to what could benefit MA.
This is one way things can work, but obviously things could be screwed up too and not work in MAs favor depending on how they would set it up. Maybe no one really wants a castle that bad and this isn't enough to coax out the players MA would want to target hoarding CLDs to exchange them for something. MA would need to figure the value themselves, but it wouldn't be as simple as just comparing a direct PED sale and buying back deeds.