You can install Eclipse (and all your extensions) in a directory which
is not writable by your developers. However when you start Eclipse it
needs some directory to store its configuration (where it writes down
which extensions are installed and which workspace directory have been
used recently). The default location for this is the "configuration"
directory in the installation directory (e.g.
/usr/bin/eclipse/configuration).

Luckily you can specify where this configuration should be stored: one
way is to use the -configuration command line option [2], but probably
in your case it's better to define it in the config.ini file in
/usr/bin/eclipse/configuration directory [1], so your developers cannot
forget it. Add the following line (check if the property doesn't exist
yet, and add it before the eof marker)

osgi.configuration.area=@user.home/.eclipse-config

Upon startup, this will cause Eclipse to create or use the directory
.eclipse-config in each user's home directory, e.g.
/users/bob/.eclipse-config. Instead of .eclipse-config you can of course
choose another name.

The same can be accomplished with the -configuration command line option:
   eclipse -configuration '@user.home/.eclipse-config'
But as indicated before, each user has remember to specify this option
(or create a startup script for it)

So as system administrator you can install Eclipse and all required
plugins with readonly access for the appropriate users, while they still
can start it because the configuration is written into their respective
home direcories.

If a user does need an extension which is not (yet) available in the
installation directory, he can install it in its own directory. To do
this he can unpack the extensions zip file (which typically creates the
eclipse/features/ and eclipse/plugins/ directories). Then he has to
create an empty file with the name .eclipseextension in the eclipse
directory (the same where the subdirectories features/ and plugins/
reside, see [3]). As an example with the CDT extension, bob's directory
structure could look like this.
/user/bob/eclipse-extensions/cdt/eclipse/.eclipseextension
/user/bob/eclipse-extensions/cdt/eclipse/features/...
/user/bob/eclipse-extensions/cdt/eclipse/plugins/...
Finally the user has to use  Help > Software Updates > Manage
Configuration > Add an Extension Location   and add the folder
containing the .eclipseextension file (in the example above that would
be /user/bob/eclipse-extensions/cdt/eclipse/).

[1]<http://help.eclipse.org/help31/index.jsp?topic=/org.eclipse.platform.doc.isv/ref
erence/misc/runtime-options.html>
[2]<http://help.eclipse.org/help31/index.jsp?topic=/org.eclipse.platform.doc.user/ta
sks/running_eclipse.htm>
[3]<http://help.eclipse.org/help31/index.jsp?topic=/org.eclipse.platform.doc.isv/gui
de/product_extension.htm>