Drivetrain squeaky birdsong Road bike with Shimano Tiagra 2x9 compact drivetrain makes a continuous random pattern of quiet squeaky noises (like birdsong) when the bike is leaning against a wall and the pedals are rotated backward. Forward rotation of pedals creates less, but similar noise. Rotating the wheels with pedals stationary does not make noise. The noise is not periodic with each pedal rotation, but its tempo increases with the rotation speed. Occurs in all gears about the same.

I wiped and lubed the chain and jockey wheels, without taking the jockey wheels out of the rear derailer cage. The noise remained about the same or slightly quieter (placebo?). Took apart derailer cage and jockey wheels, cleaned and lubed. Individually the jockey wheels did not squeak after that, but I did not check before the process. Reassembled drivetrain squeaks as before.

The chain has about 500 km on it, length of 12 links is 12 1/8 inches according to my inaccurate tape measure. The cogs and chainwheels have a lot more km, but don't look worn compared to new replacements. 

Rear derailer cage looks slightly rotated outwards (not inwards as from a crash or dropping the bike on itsright side) from the top, but looks straight from the rear ground level. The rotation axis is thus vertical, not horizontal. Shifting is fine, so I have not fixed what ain't broken.

Crankarms Shimano Tiagra. Not sure about the bottom bracket, but the circle between crankarm and frame has "BC 1.37x24 road FA" written on it. Giant OCR1 2008 bike, probably has standard components. There is no sideways play in the crankarms. I tested the bottom bracket according to Park Tool and there is no grinding or rumbling.

I took the rear wheel off and spun the cogs - no noise there.

Any other suggested solutions to the noise besides taking the jockey wheels out of the derailer cage for lubing, or replacing chain/cogs?
I suspect the jockey wheels are the culprit.  You said you lubed the jockey wheels without removing them.  Likely the bearings or bushings didn't get lubed sufficiently this way.

Typically you will need to remove them and clean and lube the bushing or pry the dust shield off of the bearing so you can clean and lube the cartridge bearing (depending o the derailleur).  If you are dealing with a cartridge bearing you need to take  care when prying of dust shields so you don't damage them.

When reassembling the derailleur ensure you use loctite (blue 242) so that the derailleur doesn't do a RUD (Rapid Unplanned Disassembly) at a later point in time.