About the only way I know of is create two custom fields. The first would be a pick list field that would contain your valid EV methods, 0/100, 50/50 etc. The second field would be a formula that would check the %complete field for a value = 0 (indicating the task hasn't started and therefore the EV for it would be calculated as 0*baselined cost of the task. Is the %complete = 100, then th eEV woudl be the full baseline value. If the % complete is between 0 & 100, then the EV would be the numerator of the EV method/100 (0 for 0/100, .50 for 50/50, .25 for 25/75, etc) multiplied by the BAC for the task.
As you can see the more EV methods you allow th emore complex the formula can get.
Hope this helps,
Dan