Solved (I hope).
Not sure how this happened, because I always allow prey node process very liberal outgoing connections, but I did the following.
sudo -u prey /Applications/Little\ Snitch\ Configuration.app/Contents/MacOS/Little\ Snitch\ Configuration
This opens Little Snitch as user "prey" rather than in my usual user
When I did this, I discovered that there were two rules (which I certainly don't remember creating) that were denying connections to prey servers and were not overriden by my rule allowing all outgoing connections from node for all users (which was supposed to allow connections from user prey) . Impt: I usually run as a non-admin, so I set the rule allowing connections for all users as a nonadmin. I don't know if things would have been different if I'd set the rule as an admin. (The vast majority of Mac users ordinarily run as admins, but the most security-conscious of us don't, so keep that in mind if you're writing documentation.)
The offending rules were:
(1) deny connections until login from node (process owner - me - i.e., prey) to ipinfo.io, port 80, protocol 6 (TCP)
(2) deny connections until login from node (process owner - me/prey) to solid.preyproject.com port 80 protocol 6(TCP)
(As user Prey) I changed both of these to "allow connections" and unchecked the "until login" box. The rules then became redundant (to my global rule allowing all outgoing connections) and now I am able to locate my device properly.
I haven't fully investigated yet, but there were a bunch of "deny" rules for the old clients when I opened Little Snitch as user prey. I can't imagine creating those myself and I suspect the new client inherited a rule from an old client.
Since the same security-conscious Mac users who use prey are very likely to also be Little Snitch users, it'd really help to have very explicit Little Snitch instructions in your documentation.