I'm afraid it's not so easy. It's much easier to change the code if you have the source available. Just add:
at the end of this.Init() in Calendar.js (line 2343).
In DayPilot Pro 5.0 SP1, AfterRender is now fired during the initial rendering as well. You can detect the callback by checking isCallBack variable (for an example, see 5.0 SP1 release notes).