Is there an Coca/obj-C API call to mimic the "Connect to Server" action in Finder? It's possible with Automater, so it seems like Finder has a hook somewhere.
Turns out there's an old Carbon function (can't find a Cocoa equivalent) called
FSMountServerVolumeSync which does what I was looking for. You can supply and
smb:// URL and login credentials.
OSStatus FSMountServerVolumeSync ( CFURLRef url, CFURLRef mountDir, CFStringRef user, CFStringRef password, FSVolumeRefNum *mountedVolumeRefNum, OptionBits flags );