Name and limit Hyper-V VM to 15 characters during OSD

We base our machine names on three characters plus the serial number during OSD using the variable OSDComputerName. When we were using MDT 2012 U1 we simply put the following into the customsettings.ini:

OSDComputername=#Left("MED%SerialNumber%",15)#

In SCCM 2012 R2 we simply set the task sequence variable OSDComputerName to the following since the above does not work:

MED%SerialNumber%

It seems that when we try to use same variable we used in MDT the task sequence will fail. Does anyone know of a way to force SCCM to prepend "MED" to the left 13 characters of a Hyper-V VM during OSD?