If you've got a custom meta column in the project panel and you use are editing the previous column and then use the keyboard tab key to move to the next column (the custom meta field), that column is selected but not you can't edit it with out clicking in it with the mouse. This is not the case with built in meta columns such as the "comment" field.
Does anyone know of any way to tab into a custom meta field and then have a further keyboard shortcut that puts the field into edit mode?
Ok, found the answer already. Tab and then tab a second time.
Obviously this seems wrong and would be better if the first tab allowed you to edit immediately.
UPDATE: Spoke too soon. This is definitely a buggy mess. Sometime tabbing into a custom field makes it editable immediately. Other times, a second tab makes the field editable, sometime the second tab just moves you to the next field. And there seems to be no pattern, I can tab and shift-tab back and forth between custom fields and the behave seems to change at random. I'll log it as another custom meta bug!