Although I am able to set l= when signing (via configuration file), l= does not processed on verifying, i.e. DKIM_PARAM_BODYLENGTH handling code does not exitst. Moreover, any mail signed with l= does not pass verification even if the body is untouched.
So, it is still impossible to send mail to the mailing lists which add something after body.
BTW, I wonder why number of characters in l= in so big in simple or relaxed mode, it seems it counts one additional char per line. In nwsp mode it is correct.