OK, I've now had some time to play with a Pioneer DEH-2800MP. I found a reference elsewhere that suggested that the Sony hard-wired remote uses the following resistance values:
0 Ω Off
2.2k Ω Source
4.4k Ω Mute
6.6k Ω List(Disp)
8.8k Ω Seek up
12.1k Ω Seek down
16.8k Ω Volume up
23.6k Ω Volume down
33.6k Ω Select
48.6k Ω Mode

This works out nicely as a series resistor chain as follows:
2.2K + 2.2K + 2.2K +2.2K + 3.3K + 4.7K + 6.8K + 10K + 15K
all nice E12 range values. These are the exact values I have found
reported elsewhere in a Sony RM-X2S wired remote device.

When I hooked this up to the tip of the remote conector jack on the Pioneer
I found the following functions seemed to work:

0 Ω Off (No apparent effect)
2.2k Ω Source (switch between CD and radio or OFF if > 2 seconds)
4.4k Ω Mute (toggles "ATT" on display with muted audio)
6.6k Ω List(Disp) (No apparent effect)
8.8k Ω Seek up (Search up-radio or next CD track - hold down to fast forward.)
12.1k Ω Seek down (Search down-radio or previous CD track)
16.8k Ω Volume up (Volume up)
23.6k Ω Volume down (Volume down)
33.6k Ω Select (No apparent effect)
48.6k Ω Mode (In radio mode switch between FM1,FM2 and MW.)

Haven't managed to get any effect on the ring connection of the remote jack
both tip and ring appear to be tied up to 5v tip (STRKEY1, pin 80 on PE5518A)
ring (STRKEY2, pin 47 on PE5518A).

Connecting various resistances between tip and ground of remote jack effectively pull STRKEY1 down to different voltages to trigger the remote control.

Anyone know what STRKEY2 is supposed to do?