That might be a little too difficult to find. A 1/4" end mill will have the ability to create way more tool pressure than the 1/8" shank might be able to cope with which would cause the bit to break easily. That type of tool might exist, but it might be very difficult to find if it does.
I would just use a 1/8" bit and let Easel cut out the hole as a pocket. Measure the bit prior to carving, and enter the value in as a non-standard bit in Easel. Measuring the bit and entering that value into Easel is the only way to get an exactly 1/4" carved hole (within machine tolerances of course.