The data directory is self-contained in /var/lib/boinc-client (older version) or if a newer client /var/lib/boinc ; all your CPID, configs, work units, graphics, etc. are all located there, so if you mount your persistent storage like that you should be OK.

Be careful though - I'm using Debian 9, the packagers attempt to make the client package more L-FHS compliant and use symlink tricks to push some of the configs over into /etc so you'd be wise to sort out how you'd like to manage that tidbit. For example, here are the symlinks they put into place on Debian 9:

$ sudo find /var/lib/boinc-client -type l | xargs ls -l

lrwxrwxrwx 1 root root 34 Apr 20 17:34 /var/lib/boinc-client/ca-bundle.crt -> /etc||ssl/certs/ca-certificates.crt
lrwxrwxrwx 1 root root 31 Apr 20 17:34 /var/lib/boinc-client/cc_config.xml -> /etc||boinc-client/cc_config.xml
lrwxrwxrwx 1 root root 43 Apr 20 17:34 /var/lib/boinc-client/global_prefs_override.xml -> /etc||boinc-client/global_prefs_override.xml
lrwxrwxrwx 1 root root 34 Apr 20 17:34 /var/lib/boinc-client/gui_rpc_auth.cfg -> /etc||boinc-client/gui_rpc_auth.cfg
lrwxrwxrwx 1 root root 34 Apr 20 17:34 /var/lib/boinc-client/remote_hosts.cfg -> /etc||boinc-client/remote_hosts.cfg

...so you have to figure out what you'd like to do to ensure you don't lose that (I for instance use custom global_prefs_override.xml tuning, so would ensure that's somehow on my persistent storage). Since they don't change all too often, a single backup of those /etc||boinc-client files should suffice - they're not runtime adjusted, just static.

NOTE: this forum seems to block "slash-etc-slash" (I can't even type it here) from all posts, I have changed it to "slash-etc-pipe-pipe" ( /etc|| ) in the above output, and stripped trailing slashes.