This seems OK, if there are some UI menu items that should be excluded we could use the OPTYPE_INTERNAL for those operators and not show their shortcuts (or add a new flag if there is some…
This adds top-level functions without to wm.py which are isolated to WM_OT_properties_edit, prefer static methods on WM_OT_properties_edit to make it clear they're spesific to that operator.
The reference to rna_enum_property_subtype_items was removed, this seems like it would be useful to keep, otherwise there is no hint that this is duplicating information defined elsewhere.
Initial review, I didn't check on the changes to _init_subtype in detail.
Why are these changes needed? They don't look related to image saving.
Prefer to keep existing logic in the case OpenImageIO isn't available (it's not a requirement for Blender on Linux distributions as far as I know) e.g.
In general having color pickers show in these small windows isn't going to well and could even result in a user clicking twice and accessing the color picker accidentally for e.g. OTOH, this…
It's not clear why this change is needed - when the repositioning is performed. (should be noted in comment)
picky use function style casts uchar(...) instead of (char).... In case uint8_t(...) makes more sense too.
As this only works on windows, better only show these items on windows.
This check makes more sense to add into the exec/invoke functions, reporting an error on failure. This way when pressing Ctrl-C shortcut while rendering - an error will show in the status bar. Also, if the user opens a menu just before render completes, the menu-item won't be outdated.