I came up with an answer to the _second_ question (how do I do this from a new instance?).
In the new instance directory: 1. Hack the system ID in the global/pg_control file to that of the original instance. 1a. Use pg_controlinfo to get the hex version of the control id: $ pg_controldata | perl -l -n -e 'if (/^Database system identifier:\s+(\d+)$/) { $sysid=sprintf("%x",$1);print join(" ",reverse $sysid=~m/../g); exit; }'
fb fb 59 56 b9 31 58 53 1b. Use a hex editor (vim with xxd / xxd -r will do) to replace the first bytes with these values.
2. The new control file will be unusable because its CRC won't match. Fix that with pg_resetlog. $ pg_resetlog -f
3. Copy the very first pg_xlog file into place. The rest can be copied by hand or found using whatever you use for the "restore_command"
4. Create recovery.conf file.
Start instance.
Again, this worked for me. What I want to be sure of is: does this really work? And I still want to know: can I do this without creating a new instance?