I had to recheck the dates in this thread. Why use 0.8.28? E-UAE 0.8.29-WIP has been available since 2006.
In any event, I haven't done it myself but it should be as easy as adding a line or two to your .uaerc file and changing permissions for the device in /dev. For example:
hardfile2=rw,:/dev/hdb,0,0,0,512,0,
hardfile=rw,0,0,0,512,/dev/hdb
And then, assuming /dev is dynamically populated by udev, for your /etc/udev/rules.d/*permissions.rules file:
# You are in the "mark" group, right? :-)
KERNEL=="hdb", MODE="0660", GROUP="mark"
If you've already tried that sort of thing without success, then I'd suggest upgrading to 0.8.29-WIP4 and trying again.