Wait, no, I spoke too soon.  The fee/reward ratio is a bit too simplistic. 
An attacker could publish one of those botnet type of blocks with 0 transactions.  But instead, fill the block with spam transactions that were never actually sent through the network and where all inputs and outputs controlled by the attacker.  Since the attacker also mines the block, he then gets back the large fee.  This would allow an attacker to publish oversized spam blocks where the size is only limited by the number of bitcoins the attacker controls, and it doesn't cost the attacker anything.  In fact he gets 25BTC with each sucessful attack.  So an attacker controlling 1000BTC could force a 40MB spam block into the blockchain whenever he mines a block.

Not the end of the world, but ugly. 
There are probably other holes in the idea too.
Anyway, I'm just suggesting that something akin to a (total fee/block reward) calculation may be useful.  Not sure how you'd filter out spammers with lots of bitcoins.  And filtering out spammers was the whole point (at least according to Satoshi's comments) of the initial 1MB limit.

I'll keep pondering this, though I guess it's more about WHAT the fork might be, rather than HOW (or IF) to do it.