The cardman driver might support OMNIKEY CardMan 2020/6020/6120 devices, but it is not yet complete, and the original author has stopped working on it.
The cm4000 driver supports OMNIKEY CardMan 4000. It is available on OpenCT 0.6.5 once released, but be aware that it doesn't currently do much more than read the ATR of a card, and you'll still need the kernel module for this card from OMNIKEY.
If you have tested one of these readers, please add a report here.
Some recent OMNIKEY USB readers should work with the generic USB CCID driver.