Full EFI reset

Question

Hello,
I have a late 2009 iMac (hardware model "iMac11,1").
I set up an EFI password with the Apple-provided utility about one year ago (or more, I was still under Snow Leopard). A few weeks later, I decided to change it, but the utility does not request the old password. So I entered a new one and thought it would be OK.
A few months later, I need to boot on an external hard drive to perform a clean install of Mac OS X Lion. I start by pressing "alt" but the external disk will not boot. So I chose it as a startup disk in the system preferences (do not do that at home). And I restart. The EFI prevents me to boot on the hard drive and requests the password to change the configuration back; so I was screwed. And of course, it does not accept the new password (I lost the previous one; yes, this is my fault).
After having browsed the web, I found the RAM technique:
shut down your computer;
remove the power cord;
remove one RAM module;
plug the power cord;
start the computer when pressing cmd + alt + P + R (all at the same time);
the Mac will make the boot sound, thrice;
release the keys and let the computer boot—the EFI should now be reset;
shut down, remove the cord, put the RAM back, plug the cord and voilà!
This did indeed work and removed the EFI password completely. I managed to select my standard disk back and managed to install Lion anyway.
But I still cannot boot on an external hard drive (be it over USB of FireWire 800) and I need it now. I think I need a full reset of the firmware but cannot find a proper way to do it.
I found pointers to rEFIt, but it is unmaintained since 2010 so this is no option.
Is there a mean to perform a full EFI restore ?
Thanks in advance for your help.
Additional information:
Model Name:          iMac
Model Identifier:          iMac11,1
Processor Name:          Intel Core i7
Processor Speed:          2.8 GHz
Number of Processors:          1
Total Number of Cores:          4
L2 Cache (per Core):          256 KB
L3 Cache:          8 MB
Memory:          4 GB
Processor Interconnect Speed:          4.8 GT/s
Boot ROM Version:          IM111.0034.B02
SMC Version (system):          1.54f36

Answer 1

Okay, I finally solved my problem: my computer would not boot over FireWire, but would over USB. I still do not understand why but I managed to do what I wanted.
As a side note, the problem happens with or without Filevault 2 activate.