I'd never heard of or used this feature before today.
Doesn't recoverydrive.exe work the same in 8.1 as it did in 8.0?
I've not compared md5 checksums, however based on the file size and timestamp I would take an educated guess and state that the boot.wim created on the USB stick when running recoverydrive.exe is in fact a renamed WinRE.wim.
I have no idea on how exactly the recoverydrive.exe works, which files uses as "source" and what (the heck) it stupidly puts on the stupid USB stick, but it shouldn't be that difficult to find out, if anyone with a 8.1 install is willing to experiment and report.
First time I tried running recoverydrive.exe it wouldn't work due to missing files. I set my (self contained) Windows 8.1 partition as active and rebooted and it worked. Taking a guess I'd say that the BCD store might be used to locate winre.wim - as this was not available on my first attempt (due to chainloading bootmgr via grub4dos - with grub4dos in it's own (active) boot partition) it failed.
It's nice to learn new things
EDIT - in terms of the missing files - I don't have a full list - or in fact any list. wpeinit.exe is missing though.