To solve my own problem, and leave an answer behind...
On the remote host, nrpe couldn't find the check_disk plugin. That wasn't built with nrpe (dunno why:?::?:) so I copied it from another host. Still didn't work. So, I manually ran it:
[root@ftp src]# /usr/lib/nagios/plugins/check_disk -w 20 -c 10 -p /dev/mapper/VolGroup00-LogVol00
/usr/lib/nagios/plugins/check_disk: error while loading shared libraries: libssl.so.4: cannot open shared object file: No such file or directory
libssl.so.6 is a symlink to libssl.so.0.9.8b, so... why not symlink libssl.so.4 to it as well? Worked like a champ! I got the same error about libcrypto.so.4, but another symlink fixed that. So I'm in business!