I posted the Customising WinPE topic some time ago, which links to instructions for manually adding WoW64 support to WinPE - along with some other WinPE customisations.
Direct link to documentation - here.
Direct link to adding WoW64 to WinPE 10.* - here
loadWoW64.exe/setWOW64.exe may no longer be required - see here
I haven't tried this resource, but I have tried your MistyPE builders, and even though the build says there is wow64 support, it doesn't actually function for me because i constantly get side by side errors running my own x86 single exe files.
now, in my personal builds using what follows, i have working x86 support in x64 pe for most every exe i've tried. i'm not sure what's missing in your work, maybe registry keys, i haven't really investigated since mine works fine now.
here is how i got my personal builds working--taken from info here and from diffing an oven build:
1. first i dism add all these packages to the standard ADK x64 winpe--in this specific order, it does matter:
2. copy all these files over the original, some are duplicates, i just let them overwrite--has to be done as "Trusted Installer" running a file explorer that is not Windows Explorer. (some may be unnecessary, i copy and replace them anyway)
3. add all these registry keys. these all come from an Oven build with wow64 support. in order to use these regfiles asis, you will have to mount the SOFTWARE and SYSTEM hives of the winpe under HKLM\MY_SOFTWARE and HKLM\MY_SYSTEM respectively, and as you'll see if you if open the files in a text editor.
4. commit your changes to the wim and then dism export your new image to a clean and max compressed wim without the [DELETED] folder.
now, if you want 1803 ADK winpe with all this already done, build x64 winpe with the ADK and use my patcher here:
Edited by stayboogy, 18 December 2018 - 01:22 AM.