I don't know how common the allowed decryption is. My only experience is with PGP Corporation that left me able to do anything, and now Symantec which controls everything at the server level and I'm not even able to access PGP Options.
If you can't boot and access PGP Desktop for the decryption, your options are pretty limited. You have the option of using the WDRT for booting, but I don't know if there is a way to somehow use this for decryption if you can't boot. You can decrypt by attaching the disk to another machine with PGP installed, or by using the WDE Recovery CD.