As of version logback-access:1.1.2, there does not appear to be MDC support.

Example:

    <pattern>%h %l %u [%t] "%r" %s %b "%i{Referer}" "%i{User-Agent}" %X{TRACER} %D</pattern>

%X is not a known pattern component, as shown in the resulting access log:

    %PARSER_ERROR[X]