Cool, thanks for all the extra explanation. I'm a developer/consultant by day, and it always helps a lot when people explain what it is exactly they're doing (trying to keep it too technical never works). :)
Actually, I built a site that was a web-based radio prep service (using a more general-purpose CMS but that's beside the point), and of course everything past the homepage was login-only, even though it was a totally free service. So, I'm a little familiar with the desire for "real" metrics in the radio world. (We also had a studio and hosted some shows, good times.)
It's not at all difficult to save the login time ... if I were doing it I'd definitely use wp_usermeta for this, rather than creating a new table (this sort of thing really is user metadata after all).
Anyway ... what you're after is a way to delete users who haven't logged in for a certain interval. But, in your case it doesn't have much to do with spam user registrations? (Or when you say "have no spam worries" are you talking about comment spam, and you do have some user spam?)
I haven't really looked at that space, but I'd guess there are a bunch of "enhanced user management" plugins out there ... at some point I could get into that (hey, maybe I will!) but I don't think that's what I want this plugin to be.
I mean, I can see how just having a last login time would be a totally useful feature to add to WordPress all on its own -- one of the things I hate about WordPress is that it does zero logging so (for somebody with both sysadmin and analytics backgrounds) you never have much of an idea what's going on with your site, unless you spend hours with grep and your www access logs. It would be a really easy standalone no-config WP plugin just to add that bit by itself.
But again, I'm not a fan at all of adding database inserts/updates to this plugin (one thing I like about it is that it runs just 1 SQL transaction a day ... there are so many bad WP plugins with so much code bloat and db overhead that they create way more problems than they solve).
(With this plugin, there's also an implementation/user support problem it would raise ... since User Spam Remover is adding the timestamp in the first place, initially none of the users have one, so when do they become OK to prune? and presumably you'd want a separate age threshold ... anyway I'm seriously rambling now.)
Anyway, since you presumably already have this Login Locker thing installed and running, I can do a quick mod to User Spam Remover for you, if you like, that joins to LL's db table and uses its last login timestamp the way you want. I mean, that's easy, probably just changing a line or two.
I'll take a look and get back to you here. In the meantime, let me know if you'd like me to hardcode a different age threshold (tell me how many days ... 30? 6 months?) for last login time or just leave it the same as the regular one.