Question: Use case is:

I type a long command and then I realize I need to run something else first.

I usually run it then CTRL C immediately.

I guess I could also do `echo "command" >> $HISTFILE`, but that won't work with quotes.

Answer: 
As an alternative to editing the history you could use `Ctrl-U` to cut the line (note that this cuts backward from the cursor, so it should be at the end), and `Ctrl-Y` to paste it back later. `Meta-Y` used after `Ctrl-Y` will cycle through the different cut values so you can get back to previous ones as well.
Just use `history -s arg`.

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> -s The args are added to the end of the history list as a single entry. 
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<http://ss64.com/bash/history.html>