I think a short forum post 2-3 days prior to the launch with an unannounced release would be the best.
Active users who want to be among the first will then know when the time has come to frequently check the store.
The post will just say that Nexus will be out in the week of the post and it maybe even reveals the price. Its not posted in the development blog post to avoid non-active users from getting an email notification. Instead its maybe shared somewhere in the Nexus forum board.
This is of course still not ideal but should give active users a very good chance to secure a XIM Nexus on launch day.
I dont think its possible to priorize very old community members without people exploiting the system or upsetting a large amount of the community. If forum registration date, post count, XIM Apex serial number, XIM Store account or other factors are considered then there will always be people who exploit their whitelisted status. Also Nexus for the most part targets a new audience and all of the aforementioned methods would lock them out. This could cause quite some uproar and negativity.