The "leeching chemicals argument" is not your concern. Anything you put your brew into is going to leave a little something of itself in your liquid. Glass is no different, or better, in this respect. The amounts and nature of the chemicals are important and for HDPE and PET these are nearly imperceptible and mostly benign. Having studied environmental toxins for the last two decades, I can tell you if you have the proper analytical tools, you can always find something. Fortunately (i think?), you encounter more (and worse) toxins filling up your car than drinking beer or mead brewed in plastic. That said, the bigger concern is that not all plastics form suitable oxygen barriers. Oxidation of your product over the prolonged storage/aging times required of most meads can occur if the plastic is not suited for the task. It would be a shame to wait so long only to be thwarted by a little O2.