This is something really common for us to change filed of master page from our body page. i also seen some asked quesions on this topic. By this document need to close this topic.
What you need to know to access master page field is that how ADS process events. Read this LiveCycle ES2 : Process Events document first. From this document you come to know that how actully our events get processed and triggered.
After reading adove document you will come to know that initialize, calculate, validate, form:ready events are triggred before master page. so we can not access master page field on this events of body page.
so it’s straight forward that we can access master page field after layout:ready event.
I’m having one TextFiled1 on MasterPage.
Using below script on bodypage on docReady event,
Hope this helps.