[...] unless SHA256 outputs are very well distributed, which they might be in this case.
The outputs of cryptographic hashes are pretty much by definition very well distributed.
I have seen on some blockchain explorers that for the genesis block the previous reference block is 0. I thought that perhaps this would mean that Bitcoin blockchain might loop unexpectedly, but I figured out that coinbase transaction has 50BTC as an output, which would I guess make it invalid now that the block reward is bellow that.
[...]
The size of the coinbase transaction is determined by the miner. In other words a miner creating such as a zero block would just decide to receive whatever coinbase transaction is currently valid.
As mentioned by RGBKey, the chance of such a collision happening is extremely small and pretty much impossible. It's an interesting thought experiment though. If subsequent blocks were to reference the genesis block (or any other older blocks for that matter) I guess they'd get orphaned due to 1) having an invalid timestamp and 2) containing invalid transactions.