Just supposing you were going to a conference tomorrow and on the programme was a possible future external examiner .... is it against all good form, good sense and good reason to approach them and ask them if they would / may just allow to you, or your sup, to officially approach them as an external examiner ?

I haven't discussed this with my sup yet at all - I've had musical sups for the past few months - so not sure of correct channels but just sounding you guys out .. how does the politics of all this work through .. iI'm sure there's the formal version and the informal version.

thanks all.

I wouldn't do it. I know that where I study at least, the external is approached informally only by the supervisor, then a formal invitation is sent by the uni.

I'd agree with previous post.  What I would do though is make sure I made some contact at the conference, ask a question (if there is Q&A at end of the session), go up to them at the end of the session, join a conversation over coffee, exchanged business cards etc.  That way you have made the connection (and hopefully impressed them) without compromising your position, then when the enquiry comes about external they'll think "Oh that was the interesting person I met at that conference - I'd like to find out more about their research" and not "never heard of them, not sure I want to do any more PhD externalling".

Win win situation.

Thanks both, sound advice.
I expect the political environment around the choice of ext examiner is rife with intrigue - i don't know why i expect that but it makes it sound quite exciting :-)  and makes it obvious i need a cup of tea.

have a good day all. 

I'd agree with Jepsonclough here, don't ask him about being your examiner, but definitely take the opportunity to meet him, introduce yourself and make a great first impression. Find out his ideas about your field and where he thinks weaknesses are in the research if possible, and see if it fits with what you are doing, then you'll have him on your side come viva time if he accepts to be your external :)

I also agree with the other posts. Asking them questions at the Q&A is a really good one!

Don't ask them directly, especailly if you haven't talked with you sup, there might be particular rules about examiners at your uni that might exclude this particular academic (i.e. not enough experience examining, etc.).
Do make contact with them and introduce yourself and your subject. I gave a paper at a conference once and afterwards an academic came up to me and offered to be my external. And generally when I mention my topic, other film historians show there interest, but (according to my sup and uni) they are not all "qualified" to be "suitable" examiners.

And there's also the "supervisor and his mates" politics: sometimes supervisors suggest their academic mates (if they research in your field, of course) to be you external if they owe each other favours (although this isn't supposed to happen officially, I think, it happens a lot. But it could work in your favour if the external is really good in their field).