Sorry to rain on your parade, but speaking as a sometime professional wedding photographer, I think you should keep the 5x4 at home and not to even ask the bride. I know it seems a bit harsh, but try to see it from my perspective..

As the official photographer at a wedding it's my job not only to get fantastic pictures (that my clients will look back on happily) but also to stage one of the most important bits of the day. Ok, It's not conducting the wedding service (I try to leave that bit to the vicar!) but shooting the group pictures is also an important "rite". It's the moment when the two families stand together for the first time after the service and it can be incredibly stressful - not just for the families involved but for the photographer too. Expectations are high and shepherding all the people, making sure they're all in the right place at the right time, is hard work and the photographer is expected to do it efficiently without ruffling any, err, feathers. (I'll leave the mother-in-law cracks to Les Dawson) It's a bit like being a sergeant major, except you aren't allowed to upset anyone.
Consequently, having someone rock up, with equipment that "upstages" mine, who expects me to do the hard work of marshalling crowds, then letting him take over "for just a couple of minutes", is something I really wouldn't appreciate. Dealing with all the uncles and their DSLR's shouting over my shoulder is bad enough, but having someone hold up the event with a 5x4 would be really, really, annoying...

Worse still, if you ask the bride if it's ok before the wedding and she in turn asks me, my answer will be "Yes, of course, what a lovely idea!", because the last thing I want to do is offend my client. Similarly, if you turn up and ask me on the day my answer would be the same - because I wouldn't want anyone thinking I was a grumpy s*d. (Yes, I know I am, but I don't want the clients to know that. Besides most brides have friends of the same age who are probably going to get married soon, too, and the last thing I want to do is put off any potential clients with what looks to them like a show of petulance).

In short, turning up with your 5x4 would really get up my nose but I wouldn't say so, I'd grin and bear it and the pictures would probably suffer ; overstressed photographers take bad pictures.

Shooting a wedding with 5x4 is a fabulous idea - but it isn't as simple as just asking the photographer/bride if it's ok. If you really want to shoot a wedding, offer to do the whole event or arrange with the bride for you to do the group shots long before the pro is engaged and then she can let him know it isn't part of his brief.