I am in the process setting a multi tenancy system up Mark and have chosen the alternative route of having a separate BS for each tenant. This actually addresses you concern as you can either have the domain (BS), Login Name and password on the same page, or, a separate login page with the domain (BS) already embedded. I intend to use the second option with subdomains
So, image two companies called, for example, ACME Clothing and MACE Motors.
ACME employees will login with
acme.myserver.com and their login will only ask for a username and password. The domain acme will be embedded in the login page.
MACE employees will login with
mace.myserver.com and their login will only ask for a username and password. The domain acme will be embedded in the login page.
That all being said.... I haven't actually done it yet but that is the plan.
In addition to that, each company will have a login to their registration app which will contain registrations of all companies and will maintain their domain (BS) company name details, billing and system config stuff.
I am let to believe, again haven't seen yet, that it is possible to get an add-on to automatically create a new BS of the name you want. Hence, when someone registers, that will fire up and create the new space, subdomain and login page and they are ready to go.
All sounds wonderful and now just have to make it happen. Conceptually it is doable.
In fact if anyone has ACTUALLY done this then you should consider offering it as a service to others (for a fee of course) as, like me, I am sure others would be happy to pay to have the tedium of working it all out (as trivial as it might be). I might even offer as a service myself in nobody else does once I am done. ๐