How to repair an Android phone where only the bootloader works?

Yesterday, I nearly bricked my phone (HTC Desire HD), but got it back up and running again. Here is how – it is a relatively simple technique but I did not find it on the web, so I thought I'd share it.

Assume your phone (any Android phone for that matter) is in the following condition:

• The bootloader works. Means, the HBOOT and FASTBOOT modes of the bootloader that appear when pressing "Volume down + Power" on a HTC Desire HD.
• Android does not boot. This makes it impossible to unlock the Bootloader with the HTCDev method, since it usually requires a firmware update first that is done with a fully booted Android and a RUU .exe file on a Windows computer. For the same reason, installing any stock firmware in its regular RUU .exe distribution format is likewise impossible. It also makes it impossible to repair anything via adb of course.
• The recovery solution does not work. For example because of a failed attempt to flash it, failing due to the "signatture verificattion" error mentioned above. So there's no ClockworkMod Recovery or 4ET Recovery Touch with Nandroid backup functionality that would allow you to restore Android to an earlier, working version. You also cannot flash a recovery solution because this is a custom flash that requires an unlocked bootloader.
• The bootloader is locked. In this case, it still had "SHIP S-ON" showing at its top, means it is not posssible to flash any custom firmware etc. via fastboot since then it will turn that down with a "signature verification fail" error. For the same reason, you also cannot use the HBOOT PD98IMG.zip method for flashing a custom image (see below).
• You cannot root or S-OFF your phone. That will be because most rooting methods require adb access or a booted Android version.

Solution: Install a stock firmware with the PD98IMG.zip method:

1. Find a stock firmware for your phone in PD98IMG.zip format. For the HTC Desire HD, I tried PD98IMG-GB3.zip from this list, and it worked.
2. Flash that stock firmware with HBOOT. That is, place the firmware to flash as PD98IMG.zip on the SD card and then boot your phone into the bootloader in HBOOT mode. It will detect the PD98IMG.zip and ask you if to flash it. And since it's a stock firmware, it will pass the signature verification. Usually, such a firmware also includes a (stock) recovery solution, so that will work too (though still without Nandroid backups …).
3. Boot into Android. Should work again now.
4. Root and S-OFF the device. Should be possible again now, since you have a working Android version again.
5. Install a custom recovery solution. I recommend 4EXT Recovery Touch. Can be flashed now, since your bootloader is S-OFF now.
6. Restore your system from a Nandroid backup. Works now, since you have a custom recovery solution that can handle Nandroid backups.