I tried this, fully expecting it to fail, but it works!
@url file://${SEAFILE}/rest/of/path/to/my_file
FWIW: I have recently switched from using Nextcloud to sync between machines on my local network, to Seafile. I have had a lot of trouble with Nextcloud in recent months; Seafile seems to do the one thing it is supposed to, with much less fuss than Nextcloud.
So on each machine (Windows/Linux) I have an environment variable SEAFILE that points to the top shared director on the local filesystem. This has been working for @file nodes, but I hadn't expected it to also work for URIs.
Hurrah and Thanks!