Have not had the chance to check this yet, so I don't know it works or not.
Taking from funkyspacemonkey com
As you know, to get into DFU you must press the two buttons simultaneously for 10 seconds, but some times one of the two buttons will break. An alternative to jailbreak without having to go into the DFU mode is to use Blackra1n. However, if the iPhone freezes and you need to restore the firmware to do it again, this guide will come back again very useful.
What you need:
original Apple firmware
BetterZip on Mac or WinZip on Windows (or something similar, your choice)
How to do it:
1. Create a copy of the original apple firmware and change the file extension from .ipsw to .zip
2. Unpack the .zip file to view its content. Navigate to Firmware > all_flash
3. Inside you will get two folders ( all_flash.m68ap.production and all_flash.n82ap.production ) and both of them will contain various files. The ones you are interested in are:
LLB.m ** ap.RELEASE.img3
NOTE: asterisks stands for 68 or 82 ? depending on the folder. ?xxx? is a different number for each device.
4. Delete the LLB.m ** ap.RELEASE.img3 file in both folders and rename the applelogo.xxx.img3 file to LLB.m ** ap.RELEASE.img3
5. Save everything and rename the file back from .zip to .ipsw
6. Connect your iPhone or iPod Touch to your computer, open iTunes and click the ?Restore? button while holding down the Alt key if you?re using Mac (Shift key if you?re on Windows).Now browsed for the modified .ipsw and restore your device with it.When iTunes is done restoring your device with the custom firmware, your iPhone/iTouch will load directly into the DFU mode and now you can restore with a custom jailbreak firmware.
So basically what you are doing here is, making LLB (which is not there) invalidate iBoot which causes it to go into a DFU loop.