I heard you mention this project at the end of your Interview with Eric on the QSO Today podcast and it immediately caught my interest as discussions of a “back channel” have been happening for a while.
I may be wrong, however my understanding of “AllStar” is that it provides an Asterix IPBX that can be connected to many (possibly all) of the various Internet based backbones used by amateur radio as well as analogue FM. I know a member of the club I am in in Australia (Ian VK2HK) has installed such a server and connected Echolink, IRLP, D-Star and his local FM analogue repeater into it, making it possible to talk between the networks. The advanced functions of the networks like finding someone by their callsign is not supported. Perhaps this is best thought of as a Teleconferencing room that can be accessed from any of the networks that an interface is installed for. In any case, this may be a direction to look at for a SOTA backchannel. An activator would of course need access to a linked repeater to be able to access this while climbing up the mountain or while operational on the summit. By having a link in from Echolink, Smart Phones as well as HTs could be used to access the “SOTA Room” as long as internet access via the cell system or a local Hamnet node.