Heat should have no affect on the diapers function to absorb the urine because babies wear diapers all the time out in the hot summer heat and they absorb just fine.
I wouldn't see any reason for the heat to affect the absorbing abilities of the disposable diaper.
The diapers have SAP that soaks up the urine and swells up.
Well actually thinking some more when you sweat during the summer heat your body produces salt from the sweat which could affect the absorbing of the urine into the sap.
In fact I remember reading that salt can actually reverse the absorbing of the urine in the SAP and dry the diaper out.
So that is a possibility but other than that just the heat shouldn't cause problems with diapers absorbing urine for either an adult or a baby.