Place the textbox inside a movieclip and mask the movieclip thats it! if that is not your problem. please give a brief expanation of your problem and why you need the mask of textbox and where you going to be utilize?
I have done that already but the mask doesn't work. I already have assigned a mask for page flipping, and other mask on other layer for revealing a textbox (while the page flips). Both masks are synchronised but it seems to have no effect for the text box.
I found here bigresource.com that you can't have a dynamic text behind a mask, so I'm wondering is that true?
Its not true!! When using dynamic textbox you must embed the font to view by the user at runtime. See if you place a dynamic textbox and in the property window, select Embed button to popup the character embedding dialog box and select as it is in the screenshot attached here. Thats it, now your mask will work without the concept of movieclip.
This is the solution for dynamic textfield for masking. It works! and I tested it.
And you mentioned that you mask is overriding by another mask and there is no other option to control it without seperating your mask into two other places or create with a movieclip.