The economic majority is defined as the will of those who actually use Bitcoin as a payment system.
No matter what the miners want, if users and merchants refuse to accept their fork, then the fork loses and cannot be considered the "true" bitcoin fork.
The problem is that it is easy to measure a miner vote. The economic majority is not so easy.
The relative value of two forks could be compared by adding a system similar colored coins.
These contracts could be added with a soft fork like the P2SH one.
OP_IF
<fork_id1> <fork_id2> OP_DROP OP_DROP OP_HASH160 <hash script 1> OP_EQUAL
OP_ENDIF
OP_IF
<fork_id3> OP_DROP OP_HASH160 <hash script 2> OP_EQUAL
OP_ENDIF
This works like P2SH and is template matching. You can have as many entries as you want.
In the example, the output can be spent on fork 1 and fork 2 by the owner of <hash script 1> and can be spent on fork 3 by the owner of <hash script 2>.
Until the deadline, the value on each fork must be preserved when spending the output. If you provide the key(s), you are allowed to consolidate entries. You can also consolidate multiple outputs to the same key even if you don't have the key.
This means that split outputs are a little more hassle to use and the transactions are larger. This doesn't matter much, since measuring the relative value of the two sub-coins only requires some of them to be traded.
If someone wants to propose a hard fork, they create a new fork id and deadline and release software that implements the hard fork at the given deadline (no miner vote needed).
To prevent spam, there could be a cost to create a fork-id (BTC paid to OP_RETURN) and the deadline could have a max time in the future (say 2 years).
After the deadline, core will allow conversion of outputs that pay to the core fork-id (probably 1) to be converted into unencumbered outputs by the person with the core-id script. Likewise, the fork software will allow outputs that pay to the fork id to be converted. Legacy bitcoin that haven't been "split" will be spendable on both sides equally.
This means that users can convert x legacy bitcoin into x fork-bitcoins and x core-bitcoins in advance of the fork.
This means that Exchanges could support trading between the two. The side that trades with the most value is the one that is supported by the economic majority.
As it becomes clear which side is going to win, the price of coins on the losing side should drop. It is unlikely that the two would stay at exactly the same value without one side winning.
Users who want to to use the losing rules are free to do so but the economic majority will have made its decision.