Two FYIs:
1. At 400mA, you're in basically no danger of heating up anything noticeably. You can hold 10V * 400mA = 4W in your hand comfortably, though in that size it will be noticeably warm. In a battery size, fuggeddaboudit.
If anything's being damaged, it will be chemically, and not apparent until you've done full charge cycles. Examples: increased self-discharge, increased ESR (even at full charge), reduced capacity, poor capacity retention over time (maybe it's only good for a few dozen charge cycles after this; or maybe a thousand, good as new??).
2. Give or take enough charge, a lead acid battery is symmetrical. Taking a reverse charge means you've
fully reduced the lead dioxide cathode (if not in bulk, then at least the active surface), and are now producing hydrogen on it, while the other is being oxidized to lead dioxide (on the surface). Lead dioxide tends to expand and flake off the surface of a bulk lead electrode, so I don't know how healthy this will be to the "sponge" structure.
So... we'll see what happens.
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Tim