Thanks for response. I really appreciate it. Documentation about ACX (which stands for Active Content Extensions) could really improve developing experience, as they are much easier to poke with than reader's Java code.
BTW, ACX are in fact
W3C Widgets (there is also doc on signing at the linked page).
reader_plugins.xml is embedded in
/opt/amazon/ebook/lib/ReaderSDK-impl.jar in new firmwares.
UPD: another
doc about Widget's signing. Amazon's keys are stored in
/opt/amazon/ebook/security/keystore_mesquite.xml. I guess, you should add your own key to that file to be able to sign widgets.
UPD2: another one
doc