If you want entities generated at a certain simulation time to have a certain attribute, the best way to do that is within SimEvents, not to have Simulink to control it via callbacks. You can do this by having the Entity Generator call out to a Simulink Function on the Generate Event Action to check the time. This MATLAB Answers post talks about how to get current simulation time using Simulink blocks while the model is executing https://www.mathworks.com/matlabcentral/answers/57825-how-to-get-current-time-of-simulation-while-model-is-executing. You can put that in a Simulink Function and access that via MATLAB in an Event Action to determine which attribute should be assigned. Here is help on the Simulink Function block: https://www.mathworks.com/help/simulink/slref/simulinkfunction.html Here is an example with a Simulink Function: https://www.mathworks.com/help/simevents/examples/simulate-scheduler-of-a-multicore-control-system.html
Please let me know if this helps.