I think this should work. Create a file myratecontrol.py
(or whatever you want to name it) in the Picard plugin folder with the following content:
PLUGIN_NAME = u'Customize rate control for own MB server'
PLUGIN_VERSION = '0.1'
PLUGIN_API_VERSIONS = ["2.0"]
from picard.webservice import ratecontrol
# Hostname and port of your MB server, 0 is no delay
ratecontrol.set_minimum_delay(('192.168.1.61', 5000), 0)
Restart Picard for this to take affect.