After 20 minutes, CPU miners should also get a smaller block reward than usual. If the difficulty drops from 1,000,000 into 1, then the total block reward (with fees) should also drop for example from 50,0100,0000 satoshis into just 5,001 satoshis. And the rest of the block reward can be timelocked into some future block number (so that ASIC miners will get it), or simply burned (which is easier to implement).
I like it: burn it! Don't keep it for future ASIC miners, that will complicate things and might still be gamed by mining CPU blocks to increase the next ASIC block reward. Just burn it. In a healthy testnet, that 20 minute timer shouldn't very often, so the few burned coins don't matter much, but at least it solved the problem of not finding blocks when a large miner quits.