There are actually various Trials "levels" that determine whether a forthcoming Trial is advertised as an "Invitation" to general community (via Announcement/News or Forum Headers) or only (initially) to a subset of existing long term trialists. This does give a "logistical and ethical conflict of interest" as majority of Trials have a NDA association and therefore one (often) can't even publicise the existence of a trial itself (unless it's via a Community invitation).
At the risk of sounding paranoid and/or unhelpful we have already lost a valued member of the SU community due to a "perceived" technical infringement of Trial T&C's whilst the member was trying to assist the general community in good faith.
E.g. The OP has not mentioned how they were "invited" to participate in Powerlines Trial but I received an invitation ( and essentially declined) via PM on 8April last year? Had the OP not already mentioned the existence of that trial then I would not have mentioned it either
If you are already "on the Trials (mini group) list" then you should see a "Trials" section on Forum homepage (albeit you may not be able to access ALL the Trials). For reasons stated I can't even publish the list on community forum(again most that I can see have been closed)
Otherwise expressing/registering your enthusiasm here may well encourage Forum Trials coordinator(s) to PM if not post further. Otherwise it's a case of keeping note of any Announcements/Forum Headers. Remember even if you respond to a Trial invitation then there's no guarantee you will be selected as the admin overhead and any hardware availability can often restrict the viable Trialist numbers and in some cases like the Public shared WiFi trials/pilots in 2015 (SH2/VMDG485) and late 2016/recent (SH2AC/VMDG490) then these are initially "area dependent" !
Regards Tony "Life is a Binary Inspired Turing Computed Hologram"(don't PM or @Mention me - in case ignoring you offends)