Hi
Having struggled with this (and other protected form limitations) myself, I found a workaround that may be helpful to some, depending on their needs. Also note that I have got this to work in Word 2007 (in compatability mode), but I'm not sure if it will work in earlier versions.
1. Bracket the place (a blank line or a table row....) where you want a hyperlink within 2 section breaks ("page layout" tab in ribbon/"breaks" under page setup block/"continuous" option from the dropdown). You can even do this on alternate lines within a table.
2. When setting form protection back on, within the "editing restrictions" block, right under where you select the "filling in forms" option, there is an option to Select Sections. Click on this and then you can choose not to protect the section where you want the user to enter their hyperlink.
This is by no means a perfect solution, as:
a) the users will be free to do what they want in the unprotected sections,
b) if you have numerous places where you want to allow hyperlinks, it can become tiresome to set it up,
c) the hyperlinks won't be 'bookmarked' as proper form fields would be (and any field inserted in the unprotected area to solve this could be overwritten).
d) you have to unprotect a whole line (e.g. you cannot create a section break within a column or cell within a table, or similar type of construct/situation, at least not that I have found out yet).
But if these cons are not critical, then this might work for you.
(On a separate note, the same approach also allows more rows to be added to a table, which is not possible if it is in a protected section, with of course the same cons as above).
Hope this helps some others.
Regards
Mark