If you open Registry editor (regedit) you should be able to find a key:
HKEY_CLASSES_ROOT\Powerpoint.Show.12\shell
Add a new key under that:
OpenAsReadOnly
Add a String value:
(Default)
with value
Open as Read-Only
and another String as:
Extended
with empty value.
Add another key under HKEY_CLASSES_ROOT\PowerPoint.Show.12\shell\OpenAsReadOnly:
command
Add a string:
(Default)
with value:
"C:\Program Files (x86)\Microsoft Office\Office14\POWERPNT.EXE" /h %1
The path to powerpnt.exe might differ for your version of Office. (It's 12 for Office 2007, 14 for Office 2010 and 15 for Office 2015.)
After that you should be able to shift+right click on a pptx file and select open as read-only.
Repeat the process with
HKEY_CLASSES_ROOT\PowerPoint.Show.8\shell
if you'd like the same behaviour for .ppt files.
Disclaimer:
You should make a backup, and proceed with caution...blah-blah-blah.
One more option if you don't want or don't have the necessary privileges to modify a registry key: Open PowerPoint, then when you select the ppt/pptx file to open, click on the little arrow on the button "Open" and there should be an option to open the file as read-only. I know this doesn't answer your question, as it's not the shift+right click menu, but wanted to include it for others who may be more comfortable with this option.