A configuration option to use Steam overlay for the URL-based MOTD would be an interesting idea (
gui.OpenURL). I'll pitch the idea to Megiddo and see what he thinks.
Otherwise, you can either modify ULX's source directly (not recommended), or just disable the ULX motd and roll your custom one.
But yes, otherwise we're stuck with a very outdated browser as Timmy has mentioned. In light of the complaints to update it, Willox has mostly implemented the latest Chromium Embedded Framework (CEF) in Gmod, but it sounds like they've hit a snag since CEF only supplies 64-bit binaries for OSX, whereas the Source Engine only runs in 32-bit, meaning it doesn't work on Mac. I do not know what (or when) solution they'll come up with.