Reduce air flow if the heat in the air is too high. There are motor controlers out that set the fan speed based on supply air temp. I do not think you will find a PTAC off the shelf to do what you want.
That's a whole lotta air!! Checkout Trane.com and look up PTAC. They claim 65cfm to 70cfm OA "off the shelf"; which is only about 18% for a one ton... perhaps the range you need for a 0.5 ton?
What size are you considering?
If it's very large, then you might want to consider an enthalpy wheel or other heat recovery device ( check out Lossnay.com)to reduce the load on the PTAC.
I just installed these on a job I designed.
What's your application; a hotel/motel or other? Buddha's advice 2,500 yrs ago (I'm Christian!!???!!): "After deep, careful observation/analysis; and it agrees with reason, is conducive to the common good: accept it and live up to it."