Sergio,

you can call this an ORBS attack or an attempt of ad-hoc coalition forming for a fork.

Preparation Step:
Include a transaction sending a sizable amount between two of your own addresses in every block.
Miner can do this at zero cost in their own blocks.

Execution:
Embed into the preferred fork a transaction double spending the regular do-nothing transaction with one that offers a sufficiently high fee. This offers inceptive to rational miner to join the ad-hoc coalition for that fork.

Attempting to form an ad-hoc coalition using above steps is open to anyone, just cheaper and easier to execute for a miner. 

Fortunately cost for (cumulative) proof-of-work creates a lower bound to the incentive that need to be offered. So your worry
of times where block subsidy is low is unwarranted as cost of POW will be high.

I do not think “disallowing” the implementation of rational mining is a viable option, since no one needs permission to implement whatever optimization he thinks is profitable and within the rules.

Tamas Blummer