You need something like this at an appropriate point in your ACL flow,
which sets an ACL variable:
set acl_m1 = ${lookup mysql{select id,mailhome from usermap where \
mailname = '${quote_mysql:$local_part}'}{$value}fail}
then later you can use the "extract" function to query that data as
follows:
${extract {mailhome}{$acl_m1}}
Graeme