An analogue to snmpd's pass_persist (http://www.net-snmp.org/docs/man/snmpd.conf.html#lbBB).

It's primarily intended for active agents and for retrieval of many user parameters at once.

Suggested setting syntax:
UserProcess=<list of comma-separated wildcards or regexp>[[*]],<command line>

Suggested protocol:
stdin<-Newline-separated list of parameters terminated by empty line.
stdout->Newline-separated list of parameter=value terminated by empty line. For LOG entries, we may require to escape embedded newlines (I haven't experience with them, dunno how much of an issue this is)
Entries missing in the output are considered unsupported. Special return values like NONE or NODATA are a double-edged sword. I would avoid them unless a real necessity arises.

Letting the program actively return data seems a bad idea to me: the pace should be controlled by Agent.