No idea if this is feasible in your case (particularly no idea what
level of security you need), but as Emacs allows you to assign a vast
choice of properties to any text in a buffer, you could use psgml
mode.
You'd only need to assign the read-only property to the parts that
must not be changed, and optionally some invisible property to hide
them completely from the user. Should not be too difficult, and
endlessly customisable (i.e. if you want to create an easy-to-use user
interface, you can go on by re-mapping keys to move about the text to
be edited).
I doubt, however, that it is possible to prevent an experienced Emacs
user from removing the text properties. Hence, this is not the same as
authentification and stuff, but it is nice if you merely want to
prevent users from messing things up.
If the text to be edited is one continuous region, buffer narrowing
might be another option.
--
Florian v. Savigny
If you are going to reply in private, please be patient, as I only
check for mail something like once a week. - Si vous allez répondre
personellement, patientez s.v.p., car je ne lis les courriels
qu'environ une fois par semaine.