Looks like the Nagios Availability-Report is generated using the cgi-bin/avail.cgi script looking at the source code for the cgi script it reads the nagios log files and generates the reports on the fly.
You could either pass the arguments to the avail.cgi script or you could parse the nagios.log file and the archives which on my system is located in nagios/var/archives to generate your own reports.
A report for all host groups for the last 7 days with first assumed state as up/ok would look like the following
http://nagios/cgi-bin/avail.cgi?show_log_entries=&hostgroup=all&timeperiod=last7days&smon=12&sday=1&syear=2012&shour=0&smin=0&ssec=0&emon=12&eday=31&eyear=2012&ehour=24&emin=0&esec=0&rpttimeperiod=&assumeinitialstates=yes&assumestateretention=yes&assumestatesduringnotrunning=yes&includesoftstates=no&initialassumedhoststate=3&initialassumedservicestate=6&backtrack=4
Just as another thought, you could download the CGI script source code and edit it so the output matches your requirement and then compile the CGI and use it.