My understanding is that pin issue. If you wanted a portable PC Engine CD playing thing you'd need to go full Ben Heck on the mess, make a new frame, drop a PC Engine GT into it, with a nicer modern screen given the effort, then cannibalize one of those TerraOnion devices into somehow a smaller form factor and mash all that into a newly made plastic comfortable handheld shell.
It would be a huge task with a huge expense, not really doable without just starting from scratch. If you were going that far into it, one would probably figure it easier to make a FPGA device that is keyed to duplicate the guts of for all things TG a SuperGrafx on a micronized scale. Add a lower power usage modern screen and speaker, plus a lithium ion battery to recharge it. And then with the added capabilities of the Terra Onion SSS3 device, yet still retain both a HuCard and a SD/MicroSD card slot for your disc dumps. But hey if you can do it, I'm sure there's a market for it. I know if it wasn't like something over $300 shipped I'd buy it as I'm more into handhelds.