The only pseudo-solution I can think of is to use a fixed-width font for the drop-down list, and then fill the content columns with spaces.
COL1_NAME | COL2_NAME
Some nice value | Another value
Another nice value| Second column
"OPTGROUP" , , .
:
, : http://twitpic.com/2lsuxx HTML
<style type="text/css">* {font-family:Courier New;white-space:pre;}</style>
<select>
<option></option>
<optgroup label=" Col1 | Col2">
<option>Some nice value | Another value</option>
<option>Another nice value| Second column</option>
<optgroup>
</select>