Wondering if EasyBCD can be used when 7 is using UEFI boot mode
According to their blog, yes, though the docs haven't been updated yet. For MBR disks it either adjusts the Vista/win7 boot loader to point to whatever Windows' loaders, or it'll optionally install the win7 boot loader & then adjust it -- e.g. if you want to start win7 without the hidden 1st partition where the boot loader's stored, or if like me you don't like the win8 boot loader & want to go back to win7's. Having it point to a Windows boot loader on a GPT disk I *think* just changes where it looks for or adds the boot loader, but I could be wrong. OTOH if you're missing some of the boot loader files because switching to MBR you lost that 1st partition, adding the win7 boot loader should put things right.
...then again I'm dying to install XP next to 7 once again
If you've got your disk(s) set up as GPT, I'd imagine the big issue would be transferring your disk/partition contents to MBR for XP to work -- I know some Paragon apps can do the conversion, & I've played around with restoring backup contents, copying partitions etc. from one to the other, but I don't think either was 100% bulletproof. The EasyBCD forums might [hopefully] have more info on switching from UEFI to MBR.
I *think* of UEFI Bios & GPT hard drives as 2 separate things -- MS docs [e.g. http://msdn.microsoft.com/en-us/windows/hardware/gg463149.aspx ] seem to hint that GPT is part & parcel of UEFI, but in my experience the bios itself doesn't think so, acts as if it's just 2 options, not unlike SATA hard drives set up as either IDE or AHCI. Switching between MBR & GPT is similar to switching between IDE & AHCI modes, except you set AHCI in the bios,while in my experience the bios sets itself to either MBR or GPT based on what it finds... the hard part is getting the partition tables etc. from the hidden fat32 GPT partition to the first, hidden MBR track.
If your bios will see a MBR VHD if/when it's in GPT mode, you also *might* be able to put XP on a VHD & boot to that, using something like the Paragon GPT driver for XP so it could see & work with the rest of your disk space once it started, but you're getting into almost uncharted territory -- I'm guessing you might have to incorporate some of the XP mods that let it boot from VHD, boot from USB etc. that you can find at sites like reboot.pro -- might have to set up a [internal or external] MBR drive as the 1st drive in the bios, install XP to that so you've got the correct drivers, back up/restore or otherwise transfer to MBR VHD, add whatever mods, then try to get it to boot.
FWIW I need XP Pro to run natively as fast as possible because there are some apps/tools that just won't run or run as well in win7/8, so I stick with MBR drives. I skip win8's boot loader because for the nicer graphics it loads graphics drivers -- for me, older versions like XP don't like those drivers being loaded into memory before it starts.
