Why not just throw the large cast iron pan onto the barbie? Assuming you have a decent Weber or something similar, you should be able to get that pan ripping hot…
It may not be the most easily done, but have you considered a flamethrower? I’m not even joking; check out the Youtube channel Sous Vide Everything to see how they do it!
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It’s a gas grill. Not sure it will get hot enough?
This has worked well for me for searing and for pizza:
https://shop.lodgemfg.com/bakeware/cast-iron-baking-pan-14.asp
It’s pricey, but effective. I throw it on the lower rack of my Vision ceramic grill will a good load of hardwood. Gets quite hot and no indoor smoke.
What make/model grill is it?
It might take a bit, but I’ve seen people using culinary torches to sear, particularly with a Searzall attachment.
All this talk is making me want to go out and get a new ceramic Egg/Kamado style grill for smoking and searing this summer! I bought my current grill used around 15 years ago and it’s over 40 years old…maybe it’s time to update!
That said - you can generate some really high heat in these kinds of grills, and I think you could sear a boatload of items for a large number of guests in a very few minutes!
Colour me green with envy. 
I discovered sous vide a few years ago (being an apartment dweller again, I needed to find a BBQ alternative). Later on I also picked up a Weber electric BBQ (compliant with our balcony by-laws here) - it’s ok for most things, but steak still needs to be done sous vide - and the sear in my cast iron pan.
Miss being able to smoke foods as well - have to be happy with utilizing liquid smoke where I can.
There’s nothing quite like the taste of a charcoal grilled steak - if you combine sous vide with searing on charcoal, I think you’d pretty much have steak nirvana!! 
What a great resource. Thanks for sharing.
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Thanks everybody! We are experimenting with several of these ideas. Your suggestions are very much appreciated.
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Sorry for the late comment, but my gas grill gets more than hot enough to sear/finish meats. Not sure on the make and model off hand, but it’s a 4 burner grill. Nothing special or expensive, it was about $150 from home depot. I did upgrade it to have cast iron grates though. With about 15-20 minutes of preheating it’ll max out the 600 degree thermometer on the cover. I’ve found it to be a good alternative when I have guests, though I haven’t tried doing individual slices. Most of what I do with a group is a large cut of meat, SV’d, seared whole, then sliced and served. It’s definitely worth a try to see if it gets hot enough. Grill marks make everything look good