What is the best and safest way to place an Arabic Shaddah (ّ = \u0651 = "ARABIC SHADDA") over a Hebrew letter?
Hebrew textbooks for colloquial Arabic need to place the Shaddah over a letter. This was done in pre-Unicode days, so either fonts were hacked to have this, or else metal glyphs were placed together (in the days of manual typesetting). Unicode, however, doesn't make it easy to position diacritical marks from one language on top of characters from another language.
I'm looking for the best way of doing this in the Unicode standard.
U+0651 is a non-spacing mark, so you should be able to place it right after a base character and it will be applied over that character. This does rely on an appropriate font and whatever is rendering the font to support Unicode correctly.
For example this looks correct to me on Google Chrome and Microsoft Edge browsers ("correct" is relative as I know nothing about Hebrew):
U+05D0 U+0651 אّ HEBREW LETTER ALEF + ARABIC SHADDA
U+05D1 U+0651 בّ HEBREW LETTER BET + ARABIC SHADDA
U+05D2 U+0651 גّ HEBREW LETTER GIMEL + ARABIC SHADDA