.DNG files are supported through the Digicam plug-in, which is actually two plug-ins: Digicam and Digiraw. In the .DNG file type configuration, instead of defining it as identical to .PEF, you'd set Load File Using to Cerious Plug-in, with in the field to its right either plug_digicam or plug_digiraw. That would allow you to choose the other plug-in for DNG files if you don't want to change the one that is being used for your Pentax files.
To fine-tune how either plug-in works, see Options | Digicam Plug-in and Options | DCRaw Plug-in (the latter is actually for Digiraw).
One of the options there is if (instead of developing the full raw data, which takes time) you want to use the embedded preview image for thumbnail creation, slideshows, viewing, etc, to speed things up. In the Digicam settings, this option is called 'Load embedded JPEG or thumbnail for..' on the General tab. In the Digiraw settings, it's named 'Quick Interpolation for'. Use that for thumbnailing and it should work much faster.
The exposure equalization that you are seeing is probably due to a DCRAW feature that causes it to develop raw images such that none of their pixels are clipped (even if they really should be, due to over-exposure). I'm not sure which of its settings affects this.. it could be 'load 16 bits per channel' or (if that's inactive) 'automatic brightness level'. You may want to experiment with those, or try plug_digicam instead of plug_digiraw for the 'Load File Using' setting.