Right now all procedure CALL statements are sent to the current master, as maxscale can't tell whether the procedure body would change any data or not. So load balancing of procedure calls can't happen (unless enforcing slave side execution somewhat manually, e.g. with "run on slave" hint filter hints)
A procedure declared as READS SQL DATA is known to only read, not write, though. So when taking such procedure metadata into account, Maxscale could send calls to such procedures to read-only slaves.