A common objection against the use of these platforms is the delegation system, where a user can delegate his vote to another user, giving rise to so-called super-voters, i.e. powerful users who receive many delegations. It has been asserted in the past that the presence of these super-voters undermines the democratic process, and therefore delegative democracy should be avoided. In this paper, we look at the emergence of super-voters in the largest delegative online democracy platform worldwide, operated by Germany's Pirate Party. We investigate the distribution of power within the party systematically, study whether super-voters exist, and explore the influence they have on the outcome of votings conducted online. While we find that the theoretical power of super-voters is indeed high, we also observe that they use their power wisely.
The Related Work section may also be of interest for you.
Are you aware of any research specifically into the risk of demotivating people who expect the ability to participate directly, but are then facing a situation where e.g. a single superdelegate can nix an initiative (this is a possibility I saw mentioned in the Jabbusch analysis)?
I'd be interested to know how much of that is just my own perception vs. what the actual data says.
In the sense of explicit research into this? Not much, especially I'm not aware of anything regarding the demotivational effect of having individual votes stand directly against superdelegate votes.
If you are just looking for information on the system in general, look for "Liquid Democracy", "Piratenpartei", "Liquid Feedback" (the name of the system used), and "LQFB" (acronym for the previous). Most of the info will be in German though.
https://www.sebastianjabbusch.de/wp-content/uploads/2011/10/... is probably the most thorough analysis you'll find, but the survey results have (according to the study itself) strong biases towards active members, the two surveys used different wording for questions so you can't make out trends, and it often commingles Liquid Democracy, the tool used, and direct participation in the party overall, which can make it hard to attribute answers to Liquid Democracy (IIRC it wasn't ever used in a binding manner).
Regarding the biases, 65% of survey participants claim they don't use delegations (page 156 by the numbering of the PDF), page 110 and following shows how high the percentage of delegated vs. direct votes was.
