apparently, there is a way to checkout a project in read-only mode.  Editing a file in this case requires an "edit" (and subsequent commit or unedit command) support:

see http://ximbiot.com/cvs/manual/cvs-1.11.22/cvs_10.html#IDX172

10.6.3 How to edit a file which is being watched

Since a file which is being watched is checked out read-only, you cannot simply edit it. To make it read-write, and inform others that you are planning to edit it, use the cvs edit command. Some systems call this a checkout, but CVS uses that term for obtaining a copy of the sources (see section Getting the source), an operation which those systems call a get or a fetch.

Command: cvs edit [-lR] [-a action]… [files]…

Prepare to edit the working files files. CVS makes the files read-write, and notifies users who have requested edit notification for any of files.

The cvs edit command accepts the same options as the cvs watch add command, and establishes a temporary watch for the user on files; CVS will remove the watch when files are unedited or committed. If the user does not wish to receive notifications, she should specify -a none.

The files and the options are processed as for the cvs watch commands.

Normally when you are done with a set of changes, you use the cvs commit command, which checks in your changes and returns the watched files to their usual read-only state. But if you instead decide to abandon your changes, or not to make any changes, you can use the cvs unedit command.

Command: cvs unedit [-lR] [files]…

Abandon work on the working files files, and revert them to the repository versions on which they are based. CVS makes those files read-only for which users have requested notification using cvs watch on. CVS notifies users who have requested unedit notification for any of files.

The files and options are processed as for the cvs watch commands.

If watches are not in use, the unedit command probably does not work, and the way to revert to the repository version is with the command cvs update -C file (see section update—Bring work tree in sync with repository). The meaning is not precisely the same; the latter may also bring in some changes which have been made in the repository since the last time you updated.

When using client/server CVS, you can use the cvs edit and cvs unedit commands even if CVS is unable to successfully communicate with the server; the notifications will be sent upon the next successful CVS command.