It'll have its uses, I'm sure, but is not like this training specialized compressors is free, or even always advantageous

The goal of OpenZL is to generate specialized compressors, optimized for user's input. To this end, you are going to train a new profile and use it to compress the data. The approach makes more sense when compressing a large flow of similar data, because the cost of training is then amortized across all future compressions.