Force OSX text field to accept copy-paste
Solution 1:
It's not really more convenient in this case, but you could run this in AppleScript Editor:
tell application "System Events" to tell process "SecurityAgent"
set value of text field 1 of window 1 to the clipboard
click button 1 of window 1
end tell
When I clicked the remember in keychain checkbox, the password was saved in the login keychain with the name of the volume, and the volume was subsequently mounted without requiring a password.
Solution 2:
I wrote a very tiny AppleScript application which allows you to paste passwords even where its not allowed by tricking the system into thinking that you're typing on your keyboard. The downside: it takes space in your Dock.
Solution 3:
Another thing that still works is to drag and drop text into the prompt. Copy from your password store, paste it into TextEdit or similar application that lets you select text and move it in a OS X native manner. Select the text in TextEdit, then click the text and drag. Drag the selected text from TextEdit to the password input field. And boom the password drops in-place.
Solution 4:
I tried the one way I thought might get around the cut & paste prohibition, which was to try using ctrlK and ctrlY, (Kill and Yank), but that also does not work.
This of course, gets back to the rationale of having a long, difficult encryption key to enhance security, but then having several copies of that key readily available in memory which can be accessed by any unprivileged process. However, there are several dozen 3rd party password managers, many free, on the App store that may meet your needs.