I've release the software I use for altitude training. I use this software with a pulse oximeter and the AltoLab altitude training system. I’m not sure how many people will find this useful, so the documentation is embryonic. http://fellrnr.com/wiki/Hypoxic_Timer and details of the supported pulse oximeters at http://fellrnr.com/wiki/Pulse_Oximeter
More background at http://fellrnr.com/wiki/Intermittent_Hypoxic_Exposure and http://fellrnr.com/wiki/AltoLab
PS, I've also added a number of pages around heart rate, such as http://fellrnr.com/wiki/Maximum_Heart_Rate which I will continue to extend and expand over time.