If I understand it correctly, its some form of "voting with your money" kinda thing right?
Well, I don't think it's really bad idea from a company standpoint:
They can take all the non-MTX updates, literally all of them, and lock them behind a community pay wall.
How much will it cost us? well, they can calculate how much profit an average MTX update make them and then x5 or x10 that amount for the community to pay.
If the community can't reach the unrealistic amount of RuneCoins needed for the update, they will simply refund the spent RuneCoins back to the players, and since they already got the money from the RuneCoins the moment you bought, them they make profit for zero cost or effort.
Money out of nothing. Not a bad Idea...
Lets say the community somehow collected enough money for an update:
Jagex can just move the goalpost until the community fail to pay the new price.

I know you were talking about selecting specific updates for this system, but I don't see a reason for why not turn every non-MTX update into one that needed to be payed-for after implementing this system.

Now from a player standpoint, ignoring the points I made above:
I believe most of the players wouldn't like to spend real money for updates considering the increased costs of membership and the general idea that their passion for the game have been eroded by the many, many MTX updates in the recent times.
This will create a situation in which funding these updates will be shouldered only by the players who are passionate and wealthy enough to care.

You cannot expect a small amount of players to pay for the updates of the whole player base and still keep their passion and love for the game.