The spot you are looking for to modify resides in the:
assets/plugins/tinymce###/
xconfig.js file
This file sets the valid,extended_valid, and invalid elements which tinymce allows/disallows in the content.
the
var tinymce_valid_elements is the defaults used, and can be modified using the var tinymce_extended_valid_elements settings like so:
var tinymce_extended_valid_elements = "form[name|action|method|id]";
note that:
1.) the order you place the elements in the var are the order in which they will appear in the form code after saving.
<form name="formname" action="page.php" method="post" id="formname"></form>
2.) the extended_valid_elements will be used for that element instead of the valid_elements if this var is modified.
http://wiki.moxiecode.com/index.php/TinyMCE:Configuration/valid_elements
http://wiki.moxiecode.com/index.php/TinyMCE:Configuration/extended_valid_elements
http://wiki.moxiecode.com/index.php/TinyMCE:Configuration
** it seems that the extended_valid_elements does not add to the valid_elements array as it states in the docs (in my tests), but ovverride it completely so keep that in mind.
** also make a backup of the files you modify just in case