I can find no mention in particular to reading a letter of DA or not, in wt rules and regulations.
However, the judicial committee is interested in only if the person requests to dissassociate himself or not.
They would be unlikely to read a letter, carefully looking to see if his new views are founded or not in the
scriptures.
They "flee" from apostasy, so would, in my opinion, be unlikely to read all the details.
Remember, they are three. Even if one were curious, he would be unlikely to admit it in front of the others.
All details would then be put in a sealed envellop and kept indefinitley, so unlikely, too, that the envellop would
be reopened later, to be read by a curious elder.