Assumption is we know who's mining for which pool. Whenever a known pool / solo miner finds a block, he also builds this altcoin block. He uses same coinbase as in Bitcoin main chain, and rewards himself with 12.5 altcoin. We can check last 24h blocks to verify this.

If this block (alt) is valid (miner/pool isn't among top 72block producing pools) btc miner who finds next block (btc) will also builds on top of this.

I checked pool stats for last 365 days.. top 50% (57.84%) are btc.com, antpool, btc.top and viabtc.

This altcoin reward can be distributed from block-0, btc genesis block.

As we use same coinbase, I think anyone can build blocks and no pow mining is required for this alt. I'm not sure.

I know this is a half-boiled solution, but I think it's quite possible. If done right, this should force hash power to distribute well among pools.

To get this reward miner has to find at least one bitcoin block. Finding BTC block is prerequisite to building this alt block.