I've had SLA's that were going bad and would noticeably overheat and were boiling off water from individual cells. I tried opening the sealed caps and manually adding some purified water, but I think that just reduced the acidic content and made things worse.
As a general rule if the battery is having problems it's usually because of something significant going on that is not good.
It might be possible to get into SLA's cells, but it's likely it won't help you much... (unless you have the correct acid and maybe an ability to measure the acidity to make sure you are getting it right)