For posterity sake:

I didn't mean this can't be done from a technical perspective.  But 
from a consistent UX perspective with context menus - i.e. the way 
right-click  (or application) menus work in all OS's that I am aware 
of - this is not something you can do.  context menus are click only.

It works as desired for me on Firefox, Chromium, and Opera. Mouse-down 
and drag to select the text, X11 puts the text in the primary 
selection buffer for middle-click paste goodness.
Apologies that I missed this. I do still prefer my implementation, but 
was regrettably short-sighted on the cross-platform front, so am very 
comfortable with the rejection.

Thanks for a speedy response.

Your patch doesn't work - since the text will disappear once you click 
on it (that's how context menus work - they disappear on click).

Regardless, this was already implemented last year:

https://github.com/horde/horde/commit/df04e69b84d0eab2ee524fdd148deb51aaeeafc4

I frequently create an email, then need to append an address to the 
CC/BCC. I flick back to the main window and quickly identify an email 
to/from the address I want. But where the address has an associated 
name, it is hidden, in a context menu, viewable by clicking on the 
button 'name of person'. Great, I can then see it, but I can't select 
the text!

Hence I end up having to create a new email to that person just to 
extract the address. The additional steps of opening and closing the 
new mail window I've found pretty frustrating over the last year, so 
this tiny patch helps my work flow a lot.