May I point you at this article:
https://www-secure.symantec.com/connect/articles/adventures-winpe-31?om_ext_cid=biz_con_cmgr_cp_20120221_pm
Although this describes a "roll your own" version of a WinPE 3.1 boot environment, I have done much the same with WinPE 4 and the methodology is pretty much identical when it comes to loading support for HTA. You will need to download the appropriate deployment kit from Microsoft.
You can then follow the process with the WIM generated via DS 7.5 to add support for HTA, etc. The script section for HTA support is at the beginning, ie
C:
CD "C:\Program Files\Windows AIK\Tools\PETools"
call copype.cmd x86 c:\winpe_x86
dism.exe /Mount-Wim /WimFile:c:\winpe_x86\winpe.wim /index:1 /MountDir:c:\winpe_x86\mount
dism.exe /image:c:\winpe_x86\mount /Get-Packages '(note - this line lists the packages already present)
pause
dism.exe /image:c:\winpe_x86\mount /Add-Package /PackagePath:"C:\Program Files\Windows AIK\Tools\PETools\x86\WinPE_FPs\winpe-hta.cab"
dism.exe /image:c:\winpe_x86\mount /Add-Package /PackagePath:"C:\Program Files\Windows AIK\Tools\PETools\x86\WinPE_FPs\en-us\winpe-hta_en-us.cab"
As for powershell support, it may well be provided by one of the other packages, such as the scripting package, but as I can't recall this for sure, check the documentation that comes with the Microsoft download.
I suspect the reason that these support packages have been left out of the Symantec DS 7.5 version of WinPE is that each package adds to the size of the boot .WIM file and consequently the size of the download required to kick off imaging.