Correct, no serial port. Unlike the E's USB, the E2 has the later Palm "Multi connector" also seen on the T|X. As you said, I don't think a cable talking to the mini-USB of the later color Garmins is possible.
As an aside, and of no use to your 60C, but of possible interest for owners of a Palm E2 / T|X / some Treo phones here is a cable for a E2 / T|X to a serial-enabled GPSr ... (blue Legend and back).... http://www.serialio.com/products/cables/PalmMulti_RS232.php