Currently the physical rewards are just intended for Kickstarter backers.
I suppose theoretically there could be excess manufactured due to minimum order requirements, which could potentially be made available, if it is feasible to do so (the Larian Vault used to have a store function, but it was shut down early last year due to changes in the EU tax laws regarding digital goods). It couldn't hurt to pay attention to announcements leading up to the release, anyway.
I'll check if there is going to be a mailing list set up.
With D:OS, the start of beta access corresponded with the release on Steam Early Access. If that is done again for D:OS 2, the timing would probably be the same. GOG has a similar 'Games In Development' feature now, so D:OS 2 could potentially be made available there, as well, depending on the details.