You can elope without even using a wedding arch just from the Sim menu, and there's that City Hall option which they showed us in their Live Stream, but I haven't seen anyone previewing the pack show us if the option for it is still on the wedding arches too. The only time anyone is clicking an arch in any of the streams I have watched is when they are already within the wedding event.
Hopefully @EmmaVane can answer that for us when she gets a chance. I'm curious too now that you have brought it up. I normally make big productions out of weddings, but it would be nice to still have that option to go to a venue and just do a "quick elopement" at the arch.