Assuming this sshd is OpenSSH v4.4 or newer, you can probably get the effect you are looking for by setting the ForceCommand option in /etc/ssh/sshd_config to something like:
ForceCommand nice -n 0 $SSH_ORIGINAL_COMMAND
The ForceCommand feature is documented here. You can also use a Match block (documented in the same place) to modify the behavior for certain ports, users, IP addresses, etc.
The main gotcha (not mentioned in the docs) is that ForceCommand runs on the user's shell AFTER the execution of initialization scripts (eg .bashrc, etc) so the lowered priority won't apply until those startup scripts complete (hopefully those aren't doing anything compute intensive...).