I just tested with the following sample code and it worked in UCI, just like is does with the legacy Web client:
function appointmentOnLoad(context) {
debugger;
var form = context.getFormContext();
var requiredAttendeesControl = form.getControl("requiredattendees");
var viewId = "00000000-0000-0000-0000-000000000001";
var entityName = "account";
var viewDisplayName = "Accounts whose name begin with A";
var fetchXml = [
"<fetch version='1.0' output-format='xml-platform' mapping='logical' distinct='false'>",
"<entity name='account'>",
"<attribute name='name' />",
"<attribute name='telephone1' />",
"<attribute name='accountid' />",
"<filter type='and'>",
"<condition attribute='name' operator='like' value='A%' />",
"</filter>",
"</entity>",
"</fetch>"
].join();
var layoutXml = [
"<grid name='resultset' object='1' jump='name' select='1' icon='1' preview='2'>",
"<row name='result' id='accountid'>",
"<cell name='name' width='200' />",
"<cell name='telephone1' width='150' />",
"<cell name='createdon' width='150' />",
"</row>",
"</grid>"
].join();
var isDefault = true;
requiredAttendeesControl.addCustomView(viewId, entityName, viewDisplayName, fetchXml, layoutXml, isDefault);
}
Hope it helps.
