I think the variation with voltage and temperature will be quite large.
I agree. I think almost all FPGAs have an internal RC-oscillator used for configuration. In MAXII (not really an FPGA but also not really a CPLD...) e.g., I know it is accessible, but is it also accessible in other families? Of course there are also people out there building ring-oscillators, but there you have almost no guarentee regarding frequency at all..
Thomas