I'd vote for the Canonet as well - and I'd suggest concentrating on getting lots of informal shots of the family and other guests.
Unless the "pro" messes up, he/she will get the usual shots. Your shots may end up being the special records of treasured memories.
Sounds like I should street shoot this wedding with my Canonet! and do a couple of quick formals with the Mamiya Super23 with 6x7back on.
I suppose portrait is what people expect at a wedding, so shooting wide and close could be quite different -and still allow me to be a guest (and not get bored!).
Update (3 weeks till wedding).
Took the Canonet for a run on Australia day put through 3 films. Pictures came out perfectly, however I found my self struggling with the awkward focus lever thing being so close to the body (guess I have big fingers).
Mean while at our camera collectors meeting I asked about my film travel problem with the Zorki (I'm officially embarrassed) the old bloke showed me the basics again of how to load a film into a camera... seriously have been doing this since the 80s... well, unlike the canonet and my old prakticas where you drop the cassette in place pull the film over and let the take up spool pick it up by its self... the Zorki which i try to load the same way but with poking the film through by my self... apparently you poke the film all the way through the slot on the take up spool first, letting it go right the way through so it doubles back on its self, then you pull the film across and drop the cassette in place. It doesn't matter that the end of the film may get caught in the take up spool when you rewind as ... ... ... the end of the film is all ready exposed from when you loaded it, and so it doesn't matter if you have to pull the back off the camera to get the last piece of film to separate from the take up spool (after you have already rewound the bulk of it) as the end is already exposed! and this means zero chance of the film falling of the spool cause i haven't stuck the end in enough! all i have to do is make sure the release mechanism for rewinding is released properly and it will work fine!!
I intend to take Zorki purely as I am comfortable using it, even shooting from the hip making good sharp pictures without people realizing they are being photographed therefore capturing the true moment. Also Zorki has several lenses I can interchange though i think wide is good for group scenes and especially candid moments. Any portrait pictures will be taken with Mamiya Press camera, and any other formal pics will be done by the pro photographer any how.
This way I can take some fun pics but not be swept away from my family -kind of like this guy from the 70s...! http://www.youtube.com/watch?v=i_Yo3FRPeQw
Well, got a little carried away, took the Zorki, Canonet QL17 and the Mamiya Super 23.
Zorki 4K got loaded with Ilford FP4, took one shot then went inside, so that was a bit of a waste.
Canonet was already loaded with HP5, this held me in good stead as a back up shooter for the fast stuff I just wasn't good enough to get with the mamiya, and produced the most consistent pictures. Was also handy as a backup shooter in the reception, once mounted with a small flash.
Mamiya was excellent, but very bulky and stressful when it came to remembering to get all the settings right quickly. However took some excellent shots that I am enlarging to 11x14 (ish).
Things I learnt. Definitely carry a small camera along with the MF. Don't carry to many film backs all loaded with different film - HP5 in MF actualy looks fantastic blown up, almost better than the PanF 50 (though I am a street photographer with natural bias!). Next time I would have two types of film, not three, and would have several backs loaded with both ready to go. Don't have a 6x7 back in amongst 6x9 backs as you will forget..! Don't get in trouble with the missus when you get invited by the pro to come along to the photo shoot off site as you are supposed to be a wedding guest not the pro's off sider... still the missus came as she new the layout of the location better than any one else including the pro (whoops!)
Thanks for all the advice guys, think I would love to do it again, but with less stress and a bit more organisation. Still I definitely got some shots the pro didn't, so once mounted I'll have a nice gift for my Bro and Sis in law.
P.s fill flash kicks Ass, but is another calculation I had to do whilst light metering setting manual speed, aperture, and remembering to winding on the film, changing back, put back in the dark side, pull out the dark side and focusing ... Damn those old pros must have been good.