27/7?
I can't see why you'd get an overcharge if you leave the battery in and the unit plugged in - I suppose my T30 spends more time plugged in than not, and with at least one battery in place (the OEM battery and the spare "Ultrabay2000" battery - unless I'm using an auxiliary drive, like the CD-RW, Zip, LS-120, Floppy, DVD, ...)
Considering that "smart" chargers for batteries can be had that can stay plugged in effectively forever, and the laptop is supposed to be a little smarter than a battery charger, I don't see as it should be any issue to leave the thing plugged in with a battery in place.
I also make an effort to keep all my batteries charged up (my T30, my wife's T20, and two Ultrabay batteries,) since leaving the battery with a full charge is generallly healthier for a rechargeable than not - a rechargeable should either be left plugged into a "smart charger," or topped off once a week if it's not being used. Even the newer battery technologies (Ni-MH and Li-Ion) that aren't supposed to be susceptible to "memory" effects like Ni-Cd are generally healthier if left fully charged - although they should be "cycled" (fully discharged and recharged slowly" about once a quarter or so (I use a high-value power resistor to discharge batteries, and the OEM charger does a good job of charging them back up.)
Anyhow, I know it's a little long-winded, but I've got a bad habit of explaining my answers (as you've probably figured out, if you've been here any length of time.) Long or not - there's my answer, generated by experience (ThinkPad 760XD, ThinkPad 600, ThinkPad T20, ThinkPad X20, ThinkPad T30, Toshiba Portege 400 or so, a couple NEC units, and a Panasonic CF41 Mark II. That I recall...)
5-90