False alert everyone - SLAPI plugins do work normally. I am using Mandriva, and, since Mandriva's OpenLDAP is built without SLAPI support, I had to build OpenLDAP myself. First time I've configured it with just "--enable-slapi --enable-bdb" (and plugins did not load), then I've tried "--enable-slapi --enable-dynamic --enable-bdb", and everything worked. Would be good if "--enable-slapi" would automatically imply "--enable-dynamic", since it isn't very obvious for an end-user.
Dimitri
P.S. For those who might be interested, I've completed an alpha version of OATH HOTP plugin, and it works pretty well. See my previous post for details.