We develop an AC Plug-in. Is it possible to place a door icon on the map in smart client? If so, can its appearance be changed, depending on door events?

When using the Access Control Module framework this is automatically included. Try to explore the Demo Access Control Plug-in sample.

http://doc.developer.milestonesys.com/html/index.html?base=samples/acplugin.html&tree=tree_1.html

See also

http://doc.developer.milestonesys.com/html/index.html?base=gettingstarted/access_control.html&tree=tree_4.html

Note also the pages under “Access Control Overview” (left hand side)

http://doc.developer.milestonesys.com/html/index.html?base=reference/architecture/ac_checklist.html&tree=tree_1.html