How does one get involved in the playtest if it's not open?
Would a "pretty please" help? :-)
I would imagine 3PP companies & developers--the Aethera Campaign Setting developers at Encounter Table Publishing, the Legendary Planet team, the d20pfsrd Starjammer crew, and likely others--are among the most likely to be brought on board. It's also likely that people who posted actual playtest data from the Ultimate Intrigue and previous playtests could be used. They may also test chunks via teams through the existing PFS network. I imagine Paizo is going with a closed playtest of select testers in order to gain the maximum usable data without having to "drink from a firehose" that previous playtest reporting on the messageboards creates.
I'm in a similar boat to you, Casiel. I've got a bunch of class archetypes, gear, cyberware/bioware, monsters, and new races that I've working on, off-and-on, since before Iron Gods was first announced. I'm really interested in how Starfinder will handle key mechanics (like running ship systems, any possible modifications to firearm rules, possible changes to radiation and vacuum exposure, etc.) so I can adjust accordingly. Likewise, there's no sense in me reinventing the cyberwheel if they're going to include it in the corebook or APs. It's early yet, so I guess we'll all wait and see.