Can you please provide me with what your doing once you are in the bootloader and also what you are seeing on the screen?
Also you can try to get the stock SPL and applying that first and then applying the RC29 Image. To do that first dl this file: http://android-roms.googlecode.com/files/G1OrigBootloader_nocheck.zip
Rename it to update.zip place it also on the root of your sdcard, and then reboot into recovery, apply the update via alt+s.
Then reboot phone into OS fully. Then reboot into bootloader and attempt to apply the RC29 Image.
At this point if your still running into issues it's probably because the phone is having issues seeing the RC29 Image.
To fix this ...
1. Format the SD card to FAT32 mode (this is sometimes needed. The image is not always found if other files are on the sd card).
2. Unzip the RC29 image file DREAIMG.nbh to the SD card. http://android-roms.googlecode.com/files/DREAIMG-RC29.zip
3. Turn the device power off and insert SD card.
4. Hold Camera button, and press Power button to enter bootloader mode.
5. Press Power button to start upgrade procedure.
6. After it finishes, reboot by pressing Send, Menu and End simultaneously.
Letme know if you still run into any issues.