Thom...
As a fellow cat owner ....
I have been in the unfortunate position that you are a couple of times in the past.
What I have found (the hard way) is that it is far more worthwhile, on a personal level, to sit and comfort and stroke your life-long friend whilst the injection is administered, and the peaceful sleep ensues with no apparent distress or agonies, than to have to look after and fortify a dear friend in support of some worthless treatment which can only extend the agony of the illness.
Oh how I wish it were legal for this to apply to us mere humans
On a musical note ....
when I have been in this position in the past I have always been haunted by this, one of my all time favourite songs, probably because of the title ....
who knows how the mind bends things ....
http://www.youtube.com/watch?v=W7mh2JW7xWs&feature=relatedSo, Thom .....
good luck with your deliberations my friend .... I do know how you feel.
There will always be friends on 'this side' of your screen (as my buddy Wyatt has always told me)
James
P.S. If you ever fancy tring a cover of that song let me know and we can do it together .... for therapy