I tend to use Nav Pro for most Foundry projects now, so bit rusty on the other options, but from memory Mega Menu will allow the inclusion of a button, which with a bit of lateral thinking could I guess launch a modal, perhaps not the default Foundry one though, not sure, would need to check.
But… And sorry, but this is the potentially annoying preachy bit… It’s not advisable to put anything in the menu bar that the user isn’t expecting. This of course is assuming your users won’t be expecting a modal!
What I mean is…
Your logo will no doubt link to the home page.
Your links without a down-pointing indicator will no doubt link to other pages.
Links with a down-pointing indicator will open a drop down.
All that is standard, it’s how most menus work.
Now, if you fire into that lot a link that opens a modal, it’s not expected, so potentially bad practice and worse, confusing to users. And confused users tend to leave sites fast.
I know, that was a bit preachy, and it might be you know exactly what you are doing: There will always be good reasons to go against the norm. But I kinda figured it was worth mentioning, just in case.
Yes, I know exactly what you are looking to do, and while many sites do this, most, or at least the ones that have thought about things, also have a dedicated log in page. Why? So people can navigate directly to it.
I use Sitelok and a dedicated log in page would always be my recommendation. I use it on the Template Repo page, and it’s a dedicated log in.
The reason for the dedicated login is so I can put direct links to the page elsewhere on the site.
If you really do want to go the modal route, I’m pretty sure Sitelok includes a snippet that you can attach to a button to open a login popup. Perhaps looks at that with the Mega menu.
When it comes to UX, I vote for dedicated login and registration pages. If ppl have to login regularely, they love to bookmark this page.
But this doesn’t mean that you shouldn’t not offer other ways of login. It’s also a common pattern to have the login form directly in the main navigation, especially if the website requires to be logged to be usefull (web portals, membership sites, gaming sites…)