The 'serial cable' that comes with the unit and which is used for kernel and software upgrades, is as you have discovered, a cross-over cable [i.e. a DTE to DTE cable also commonly called a 'null modem' cable].

The output pins of the serial connector on the docking sled are not crossed over (and this connector appears to be a DCE device).

This means you can plug the docking sled cable straight into the PC serial port as its a DCE device connecting to the PC serial port (a DTE device) so no cross over cable is needed (as you've found).

So, what you need is a straight through 'extension' cable (also called a DTE to DCE straight through cable) with a 9 pin male on the Docking sled end and a 9 pin female on the PC end.

However you should also note that one of the pins on the Docking sled connector outputs +12 Volts [this is Pin 9 I think], which may be confusing (but should not be harmful) for your PC, as pin 9 is normally incoming Ring Detect and that toggles up and down to inform the PC that you have a incoming call.
If you find that your PC (or more likely, the software you use to communicate with the unit) doesn't like the straight through cable then this may be the reason and you may wish to buy or make up a bare bones straight through cable that connects only pins 2, 3 & 5 to remove the 12 volts pin from the interface that the PC sees.

Also note that according to Altman [Hugo] docking sleds for units earlier than about serial#1050 had a wiring problem in that earth wire (pin 5) of the serial connector on the docking sled was not wired up/was missing - there is a comment about this on the BBS elsewhere and possibly in the FAQ and a workaround.

If you Empeg/RioCar's serial number is higher than about #1050 then this won't be a problem.