As per my understanding we capture the value of the Height as a numeric rather than in units and you can configure the units at OpenMRS concept level. We are following unit convention only when we calculate BMI status in BahmniObsValueCalculator.groovy file. And you can change the units in groovy file as per your need.
https://github.com/Bahmni/default-config/blob/master/openmrs/obscalculator/BahmniObsValueCalculator.groovy